Is Darkness an effective spell for making someone's approach less likely to be noticed in a dark environment?

Question

While brainstorming ways to sneak up on someone, one idea was to go at night and cloak our infiltrator (who can see through magical darkness) with the Darkness spell. The expectation was that being covered with Darkness would act as camouflage, causing them to blend into the night.

Further reflection led to this counterargument: Any watcher without darkvision can't see in the dark anyway, so Darkness would add no benefit when trying to evade that watcher's notice. However, if within the visible range of a watcher with darkvision, that person would see a 30-foot-diameter sphere of blackness maneuvering through what, to them, is a dimly-lit space, which would surely attract more attention than not casting the spell at all. Therefore casting Darkness has no benefit, and may in fact make things worse.

Is this interpretation correct, or can Darkness actually provide a benefit to someone who wants to escape notice?

Answer 1

Whether it's a disadvantage depends whether your DM assumes people have perfect knowledge of their own vision range. Advantages would be situational.

Yep, with full system knowledge, Darkness is pretty useless for the purpose of avoiding someone noticing someone is sneaking around. If you don't have darkvision, or the sneak is outside the range of your darkvision, you either can't see them regardless (it's just magical darkness on top of non-magical darkness, and you can't see through either) or there's a suspiciously extra-dark spot in a spot you know you could otherwise see.

That said, I'd make the case that this wouldn't be all that noticeable as long as you're not getting too close to the observer. D&D draws "bright" lines for how far darkvision extends, and how far various sources of light produce bright/dim light. But in reality, at least for light sources, it's a gradient; a torch doesn't have 20' of perfect lighting, then 20' of inadequate lighting, then no light at all beyond 40', there's a gradient, and the fixed distances are a rough gauge of where the different rules come into play. Presumably a similar thing occurs with darkvision; if you start walking backwards from your friend in total darkness, you don't go from "seeing them dimly, but well enough" to "seeing them not at all" as you cross the threshold of your darkvision, they're just "dimmer, dimmer, dimmer, oh, huh, I can't even see them waving their hand anymore now".

The transition may occur, for rules purposes, at X' away, but the character doesn't experience it as a binary "see/can't see" state, they just see stuff fading at a distance. Something fading to complete blackness a little closer than normal isn't something I'd bet most characters would notice, unless they were expecting a magical darkness-assisted intruder. If the observer is moving around, especially in territory they're not familiar with, simply having a dark area continue to be dark as they get closer to it would be pretty subtle; things you could already see becoming dark might be suspicious, but stuff you couldn't see remaining dark when you get a little closer would be less noticeable (it's not like you saw what was there when it was outside your darkvision range, so you have nothing to compare to).

It might be a problem if they get really close; if you rule that Darkness blocks vision things they used to see around the darkness (moon, stars, a torch in the distance) become obscured as they get really close (the darkened area filling more of their field of vision), while if it doesn't block vision, when their darkvision range can pick up stuff on the other side it would be weird to see little/nothing in-between.

All that said, while the disadvantages should be pretty minimal (is it really worse for them to detect your darkness, when the alternative is that they'd probably see you directly?), there's little benefit to be had here if you're just sneaking around, not attacking, and being detected means you lose. Dim light, by itself, imposes disadvantage on Wisdom (Perception) tests that rely on sight. Darkness just ups that to the Blinded condition, which means the observer "automatically fails any ability check that requires sight," but since they can also rely on hearing to detect you sneaking around (unless you tack on Silence), it's disadvantage on the perception checks either way.

The benefits accrue when:

1. You're planning to attack your target. Without Darkness, you'd rely on stealth to hide, it would depend on your DM how easily you could make an approach while remaining hidden, and once you attacked, you'd lose the benefits of being hidden (namely, advantage on attacks, disadvantage on opponent attacks, denying them knowledge of your location). With Darkness, you're not worse off on your approach (if you approach head-on, the darkness blob gives away your approach, but so would approaching head-on without darkness), and since you can see through it and they can't, you'll get advantage, and they'll get disadvantage for as long as the darkness persists. Plus, you can re-hide essentially at will, which could be helpful for a rogue making hit'n'run attacks.
2. You need to avoid being ID-ed. If you lose the Stealth vs. Perception contest, the observer is likely to get a look at you. Not necessarily a good look in dim lighting, but maybe enough to ID you if they already know you well enough. With Darkness, worst case, they know someone was sneaking around, but not who was sneaking around.

Answer 2

Your interpretation is correct

You’ve already laid out all the relevant arguments by rules as written yourself. Sometimes the game mechanics can result in counterintuitive outcomes, and the rules for darkeness in particular are a bit problematic, as explained in detail here. (Another well known example related to vision rules being unintuitive is two blinded archers shooting at each other from a distance hitting as if they could see each other.)

There can be other advantages of operating in darkness of course, but for the purpose of evading detection it does not help here.

That does not mean that your DM cannot rule otherwise, if you all prefer another interpretation.

Answer 3

While a constant 30' sphere of essentially pitch black darkness that only special senses allow vision through could be a give away when an observer enters it and notices they've essentially gone blind can set off alarms in their head. That being said when doing espionage and wanting to avoid detection this combo can work well if the character is versatile enough.

If you're walking through an Elven King's palace and are trying to avoid being seen by his Elven guards who all have darkvision they might set of alarms sooner than a Human because their friend within their Dimly lit vision range suddenly disappeared, but a neat part about Darkness is that it itself can be concealed. Put it on a picture inside your locket and open and close it when you need to. You have a way to render yourself unseen allowing you to escape from harm or possible detection.

While Darkness is not the best way to avoid detection it is one of the few ways to do so at lower levels. You have the spells Fog Cloud, Darkness, and Invisibility each of which have positives and negatives. However one benefit Darkness has over these other two options is the ability to possibly cripple on lookers.

Invisibility breaks when you cast a spell and Fog Cloud is pure Heavy Obscurement. But Darkness is Darkness but Magical. It requires concentration, but has no other penalties save for not allowing Darkvision to see through it. So if you wanted you could cast Prestidigitation, Thaumaturgy, Control Flame, and other similar spells/cantrips to snuff out torches, sconces, open flames, and similar light sources to aid yourself or infiltrator.