Does the Mind Blank spell make you immune to madness?

Question

From the Mind Blank spell's description:

The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

Hence, a creature under the effects of Mind Blank should intuitively be immune to Madness from p. 259 of the DMG.

As answered by "Does the Mind Blank spell prevent the target from being frightened?", however, unless an effect explicitly targets the mind (as Feeblemind does) or other points mentioned in the spell's description, it seems like Mind Blank would offer no protection against the effect (e.g the aforementioned question answers that Mind Blank does not protect against being frightened). Certain types of madness lie within this category, such as:

31-40: The character becomes frightened and must use his or her action and movement each round to flee from the source of the fear.

From p. 39 of the Curse of Strahd adventure, however:

The Mad Mage has forgotten his name and the world whence he came. In fact, he doesn't remember anything that happened before the madness... Under normal circumstances, a greater restoration spell cast on the Mad Mage would restore his wits and end the madness... But in this case, the Mad Mage has cast a mind blank spell on himself. As long as that spell remains in effect, his sanity can't be restored by any spell.

There are two interpretations of the above example:

1. A creature's current sanity/madness level is an aspect of the mind protected by Mind Blank. Therefore although certain effects of madness may not explicitly target the mind, so long as it is a type of madness it is protected against by Mind Blank.
2. Mind Blank does not protect against all types of madness, but the above example happens to be one of which that is protected since the text explicitly details its interaction with Mind Blank.

As the above is the only example I know of where madness and Mind Blank interact, I have very little data to go on.

Are there any other examples from published sources or rules I missed to help explain whether Mind Blank protects against all types of madness?

Answer 1

Mind Blank may not make you immune to madness, but it blocks all effects that cause Madness.

1. CoS Example

As your example mentions in CoS, Mind Blank prevents all spells that affect the target's current state of mind. The key point is separating the idea of immunity to madness from immunity to effects that cause you to go mad. The reason the character remained insane was because he did not have Mind Blank active at the moment he became mad. He only casted Mind Blank after he went mad because the madness made him:

suffer from the paranoia that powerful enemies are hunting him

If Mind Blank made you immune to madness, he should have been cured as soon as he casted Mind Blank. Remember, however, Mind Blank's description does not mention that it cures madness, it merely states that the spell prevents anything from further affecting their mind. In essence, the spell preserves the target's current state of mind. If Mind Blank did not protect the target's current state of mind, then the text would not explicitly state that:

As long as that spell remains in effect, his sanity can't be restored by any spell.

Therefore although Mind Blank does not make you immune to madness, any effect that would cause it or cure it is prevented by Mind Blank.

2. RAI: If it Quacks Like a Duck...

This idea that madness is a state of mind is further reinforced by the definition of "Madness" according to Merriam Webster:

a state of severe mental illness

Many effects that induce madness not only require wisdom/intelligence saving throws, it also deals psychic damage. For example in IDRotF page 174:

Anyone other than Xardorok who touches the statuette of Deep Duerra must make a DC 16 Wisdom saving throw, taking 18 (4d8) psychic damage... Along with the psychic damage comes a terrible sensation of having one's brain devoured by an illithid... Anyone who keeps the chardalyn statuette on their person for more than 1 hour gains a random form of indefinite madness.

As mentioned in Does the Mind Blank spell protect against the secondary effects of the Tasha's Mind Whip spell?:

it deals psychic damage and is resisted by an intelligence saving throw. If that isn’t affecting the mind, I don’t know what is.

Therefore madness is not only colloquially defined as a mental state, effects that cause the state to present itself deals psychic damage and requires mental saving throws. As the old adage goes, "If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck." These all imply that you must affect the mind to inflict madness.

3. Madness as Described in DMG page 259

The strongest argument for this interpretation, however, is probably the Madness descriptions in the DMG, where inflicting madness is stated to be imposing something upon the mind explicitly (i.e affect the mind).

Various magical effects can inflict madness on an otherwise stable mind.

Later descriptions also mention keywords that supports this interpretation:

Some artifacts can also break the psyche of a character who uses or becomes attuned to them.

As DnD also uses natural language (so that sourcebooks and adventures do not read like your programming textbook), the mentioning of "breaking your psyche" also reinforces the interpretation that inflicting madness is a processes affecting the mind.

In conclusion

Though Mind Blank does not make you immune to madness, if no effects can cause you to go mad, it basically serves as effective immunity (so long as you are not already mad before casting the spell). It essentially protects your current state of mind. You remain sane if you were already so when you cast the spell, and likewise if you were already insane as seen in the CoS example.

Answer 2

Mind Blank does not generally block madness

I have a total of four different reasons why it would not -- if any of them holds, then Mind Blank does not in all cases block madness.

1. Proof by Example

The example you cite from Curse of Strahd is in fact a proof point that Mind Blank does not generally protect from Madness, especially not from internally created madness driven by horrible experiences of failure: the Mad Mage is explicitly insane

he doesn't remember anything that happened before the madness...

and yet

the Mad Mage has cast a mind blank spell on himself

If Mind Blank would protect from madness, he could not suffer from madness.

2. Style of Play

Secondly, the madness rules are to support a different style of play, more in line with something like Call of Cthulhu, where you explore human psyche's reaction to facing unnatural horrors instead of just shrugging it off like typical D&D adventurers:

In a typical campaign, characters aren't driven mad by the horrors they face and the carnage they inflict day after day, but sometimes the stress of being an adventurer can be too much to bear. If your campaign has a strong horror theme, you might want to use madness as a way to reinforce that theme, emphasizing the extraordinarily horrific nature of the threats the adventurers face.

If you could just Mind Blank yourself to be entirely immune to any cause of madness, this would undermine this style and atmosphere of play. When the use of madness is a DM meta-decision about the kind of campaign they want to play, it cannot be invalidated by a single spell.

3. Artifacts

The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

The madness rules say

Some artifacts can also break the psyche of a character who uses or becomes attuned to them.

Artifacts in general come with their own rules that cannot automatically overcome even by a wish spell. They are more powerful than wish. For example, you cannot just use wish to destroy them, you need to find out their secret method of destruction. So Mind Blank will not be powerful enough to foil them, if "wish and effects of similar power" is the extent of what it can overcome.

4. Rules Text lawyering

This argument may actually be the least relevant, and I think the main thesis can be supported without it. Also the amount of weight put here on exact wording makes it a bit tenuous. Mind Blank states

The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

It will protect you from mind affecting spells or similar effects used to affect your mind, but madness can have many sources other than magical effects. The DMG on p. 259 says:

Diseases, poisons, and planar effects such as psychic wind or the howling winds of Pandemonium can all inflict madness.

Diseases, poisons and planar effects may be naturally occuring and not be something used to affect your mind. They therefore would not be affected by Mind Blank.