Core:Disadvantage

Sometimes an effect will specify that you have Disadvantage on an attack, feat save, resistance save, or some other dice roll involving a single die. When this is the case, you roll that die twice and take the lower of the two rolls. If multiple sources would impose Disadvantage, you still only roll the die twice. If you have both Disadvantage and advantage, then you roll normally. This is the case even if you have different numbers of sources of Disadvantage and Advantage.

If you are blinded, then you have Disadvantage on attack rolls against creatures that you can't see. If you make an attack, then you roll 2d20kl1 instead of the normal 1d20. If the creature you're attacking is also blinded, then the Disadvantage from being blinded and the advantage from them being blinded cancel out, resulting in a normal 1d20 roll.