How does picking an origin for AOE spells work in an area that you cannot see?

Question

The general question is: How does line of sight and blinding conditions affect area spells?

Specific questions:

• Can I target the point of origin of an area spell, like fireball, in a zone where I do not see, a dark room or a zone affected by a darkness spell?
• If I play with a square grid, am I allowed to pick exactly the point of origin on the mat?

In my opinion picking the point requires precision which would require at least a perception roll, but the rules are not clear on this.

• Is there any kind of penalty for a blind caster casting an area of effect spell (like there is for spells that require an attack roll)?

The only relevant paragraph I found on the PHB is "A Clear Path to the Target" which does not answer my question.

Answer 1

Well, to start with, page 202 of the PHB, under Range, says:

The target of a spell must be within the spell’s range. For a spell like magic missile, the target is a creature. For a spell like fireball, the target is the point in space where the ball of fire erupts.

So there's that. Then on the next page, under Targets, we have:

To target something, you must have a clear path to it, so it can’t be behind total cover. If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

This is interesting, because it clearly indicates that it is possible to target area of effect spells on locations that you can't see. If it hits something on the way, the area of effect will be created there, but otherwise, go nuts.

This is all that the rules say about targeting AoE spells. It's up to you and your DM to decide, in a specific situation, whether you would be able to target a specific point.