Here’s the setup:
My familiar is adjacent to a target.
I use my action to use my familiar’s senses. I’m now blinded and deafened.
I use a bonus action to cast a touch spell. My familiar uses its reaction to deliver the spell.
Normally, being blinded causes you to attack at disadvantage, but I can use my familiar’s senses.
Do I attack at disadvantage, or not?
Casting while blinded in D&D 5e (mentions casting while blind, but not specifically the issue of a familiar delivering a touch spell)