Here is the scenario:
A warlock casts darkness (level 2) on himself, then a cleric casts dispel magic, targetting the "magical effect".
Since dispel magic reads: "Choose one creature, object, or magical effect within
range. Any spell of 3rd level or lower on the target ends", the target is valid. The problem is that dispel magic doesn't specify the area of effect. Doesn't the target of the dispel magic spell have to be the creature on which the darkness spell is cast? or can it be dispelled like an AoE dispel effect?
Best Answer
The area of effect of dispel magic is already describe in the quote you used there: “one creature, object, or magical effect…”.
The point of confusion appears to be the idea that an AoE magical effect “must” need an AoE dispel magic to be properly targetted. But that's not the case: AoE magical effects are still singular magical effects and can be dispelled by a single-target dispel magic.
(Dispel magic would only need an “area” target description if dispel magic could affect multiple creatures, objects, and magical effects in a single casting, but it can't, so it doesn't.)
So if darkness was cast on a spot, it is an independent magical effect which you can target and dispel magic will affect the darkness spell itself. If darkness was cast on a creature, then you could target your dispel magic on the creature or the visible magical effect, but in practice you would always target the magical effect due to the difficulty of even knowing whether there’s a creature there to validly target.