If both attacker and defender are blinded it's a straight up roll, not disadvantage as in current game state.
Official rule text:
> Blinded
> 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).
Bumping this because it seems to still not be fixed in the Spring update. I had cast Fog Cloud in order to ensure no-one had any sort of advantage or disadvantage (a common tabletop tactic) and was surprised when everyone just had disadvantage. This also might be related to the issue of advantage and disadvantage stacking when they shouldn't.