The Archmage NPC casts Mind Blank on itself before entering combat per the MM. Mind Blank is powerful enough to foil a Wish spell to gain information about or affect the mind of a target, per the PH.
How does this affect a Zone of Truth spell? Would the Mind Blanked creature have to make a saving throw? Would the Zone of Truth caster be aware of a failed save? Would the Mind Blanked creature be aware of the Zone of Truth?
Zone of Truth (PHB, p. 289):
Until the spell ends, a creature that enters the spell’s area for the first time on a turn or starts its turn there must make a Charisma saving throw. On a failed save, a creature can’t speak a deliberate lie while in the radius. You know whether each creature succeeds or fails on its saving throw.
Mind Blank (PHB, p. 260):
Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition.
(emphases mine).
Zone of Truth isn't a divination spell (it's an enchantment) and doesn't allow the caster to sense emotions or read thoughts. Instead it magically restricts what an affected creature can say; rather than detecting lies, it just prohibits them from being spoken. The effect that lets the caster of zone of truth know whether a target has saved or not also doesn't meet any of these criteria. So, mind blank confers no benefits of any kind when being subjected to a zone of truth; a mind blanked character still needs to make a saving throw, and is subject to the effects of the spell if they fail.
Would the mind blanked creature be aware of the zone of truth?
An affected creature is aware of the spell and can thus avoid answering questions to which it would normally respond with a lie. Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth.
(Zone of Truth, PHB p. 289)
Any creature affected by zone of truth is aware of it; again, mind blank doesn't change this.
Based on the wording of mind blank, zone of truth fails. Mind blank specifically says (PHB, p. 260):
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.
Forcing the creature to tell the truth would definitely count as affecting the target's mind and is immune to the effect. Also, I don't think the caster would have any idea that the target succeded the saving throw because that would be gaining information about the target.
The main effect of Zone of Truth (PHB, p. 289) is the following:
On a failed save, a creature can’t speak a deliberate lie while in the radius.
The spell Mind Blank does the following:
Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition. 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.
Mind Blank doesn't protect against an effect preventing speaking lies, so the main effect of Zone of Truth is not affected.
However, Zone of Truth also does the following:
You know whether each creature succeeds or fails on its saving throw.
This effect is blocked by Mind Blank, as it is used to "gain information about the target".
Mind Blank protects against Zone of Truth.
From Mind Blank:
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
And Zone of Truth preventing target from lying is a form of mind effect, so Mind Blank protect against it. Plus the Zone of Truth's caster wouldn't know if target successed or not as it would be a gain of information against which Mind Blank also protects.
I would say that the Zone is rendered useless as it is a spell used to gather information and affects the mind (page 260 PHB).
