If a spell like darkness or fog cloud covers two melee combatants, effectively not much happens. They both suffer the blinded condition, but as that gives them both advantage and disadvantage on hitting the other guy, both roll normally.
But what happens if one of the combatants is in the zone of darkness, at the edge, and the other combatant is just outside the zone. The guy inside is still blinded and has disadvantage on hitting the guy on the outside. As his target doesn't suffer from the blinded condition, he doesn't get advantage to counter that, and has to make his roll with disadvantage. But what about the guy standing on the outside? I can see how he might get advantage because his target is blinded. But does he also get disadvantage for striking into the darkness / fog, where he can't effectively see his target?