Setup
Adventurers L and R are each in an area of bright light.
Adventurer M is between the two, out of range of both light sources.
R has darkvision with enough range to reach M.
No-one has blindsight.
Normal Darkness
This does not block vision.
L and R can see each other clearly and attack each other normally.
M can see L and attacks at advantage (target can't see attacker).
L cannot see M (M is effectively invisible to them) and attacks at disadvantage (attacker can't see target), after guessing a square to attack.
M can see R and attacks normally.
R can see M (because of darkvision) and attacks normally.
Note: M might be silhouetted by a light source. In this case, L has a clue to which square M occupies.
Obscurement
For example, the gray area is a fog cloud spell, or a sand storm, or heavy foliage.
This blocks vision but does not block the existance of light ("You can see the shifting light of a bonfire on the other side of the trees.").
None of L, M, and R can see each other. All attacks are made at normal (disadvantage from "attacker can't see target" is countered by advantage from "target can't see attacker"), after guessing a square to attack.
Magical Darkness
For example, the grey area is a darkness spell.
This completely blocks vision (the "inky blot" concept).
None of L, M, and R can see each other. All attacks are made at normal (disadvantage from "attacker can't see target" is countered by advantage from "target can't see attacker"), after guessing a square to attack.
No trace of light penetrates the magical darkness. R could have a bonfire the size of a house but L and M won't see it.
