If both attacker and defender are blinded it's a straight up roll, not disadvantage as in current game state.
Official rule text:
> A blinded creature can’t see and automatically fails any ability check that requires sight.
> Attack rolls against the creature have advantage, and the creature’s Attack rolls have disadvantage.
So the attacker has disadvantage for being blind, but advantage for attacking a blind target. Whenever you have both advantage and disadvantage it's a straight roll (regardless of the number of sources of each).