The Player Character (a Sorcerer/Warlock) is talking to an enemy while being blindfolded. Therefore he cannot see the enemy, but obviously hear him. Which spells can he cast?
Ranged Spells that require an attack roll
For example Scorching Ray. I assume he would have disadvantage, because he is effectively blinded, but he could try.
Spells on himself
For example Mage Armor. I see no problem here.
Spells that require touch
For example Vampiric Touch. If he's already touching the enemy, he could cast it. Otherwise he would have to make a succesful melee attack, again with disadvantage.
Ranged Spells that do not require an attack roll
For example Sleep. I'm not sure on this one. Can he just cast it?
I would assume that he cannot cast Magic Missile at all, as it requires him to see the target. Or can he just shoot the missiles blindly and hope to hit something?