If the Warlock is inside, NO. If they are outside, Maybe.
The real key here is the wording on Hunger of Hadar. The spell breaks itself down into being inside the sphere, and being on the outside looking in. We'll use that same breakdown as well.
(emphasis mine)
A 20-foot-radius sphere of blackness and bitter cold appears, centered
on a point with range and lasting for the duration. ... No light,
magical or otherwise, can illuminate the area, and creatures fully
within the area are blinded. - *Hunger of Hadar, Player's Handbook
Inside
If you are fully inside of the sphere you have the blinded condition. Devil's Sight does nothing to counteract that condition.
“A blinded creature can’t see and automatically fails any ability check that requires sight
Outside
This is where the RAW gets fuzzy. As Justin T mentioned,it would be a DM discretion issue on "blackness" being the same as "darkness". Within the text of Hunger of Hadar it mentions opening "a gateway to the dark between the stars". This uses the word dark, so I would probably rule that you can see into it. At the end of the day though, it would be up to your DM.
Breaking your overall question down you ask:
Would someone with darkvision or Devil's Sight be able to discern those lighting differences we consider important to our normal vision?
Would someone with darkvision or Devil's Sight be able to detect where subtle shadows are when there's a faint light source to cast them?
Do the rules have anything to say, or is there guidance from closely connected material or other editions available?
Darkvision
A discussion of the difference between these the rules describing darkvision can be found here what is the correct interpretation of darkvision and the rest of this answer will assume that darkvision is consistently applied as written in the race descriptions (e.g. dwarf PHB p.20) though it actually does not make much difference to the answers.
Darkvision PHB p.20:
You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
There are two specific levels of light covered here:
- Dim light, where there is a source of light
- Darkness, where there is no source of light
In both cases darkvision enhances vision such that "you see as if it were [the next higher level of lighting condition]". Obviously you can also see at all light levels in between using the same "as if" rule and there can be no shadows if there is no light source. As always the light level present are adjudged by the DM.
Addressing your questions one by one:
1. Would someone with darkvision be able to discern those lighting differences we consider important to our normal vision?
Using the "as if" rule YES, with the exception that colour is replaced with shades of grey in darkness. If a creature with normal vision would be able to discern the lighting difference in the "as if" lighting condition, then so can a creature with darkvision in the actual lighting condition. Otherwise they can't.
2. Would someone with darkvision be able to detect where subtle shadows are when there's a faint light source to cast them?
The question appears to be asking "would you be able to see subtle shadows if the lighting conditions were less than dim but more than darkness?"
If the DM adjudges that a creature with normal vision in the "as if" light level would be able to see the subtle shadows in question then YES, a creature with darkvision can see them in the actual lighting conditions. There is obviously no sudden jump between darkvision's benefits in dim light to those in darkness, it is continuous.
Devil's Sight
There is a discussion about how the Warlock ability Devil's Sight works here.
Warlock's Devil's Sight (PHB p108):
You can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.
This lets you see in total darkness as if it were full light. You gain no bonuses in dim light from Devil's Sight so the answer for darkvision above applies. However for full darkness you get "normal" sight. So it is full colour and any enhancements (goggles of minute seeing) also work. However as there is no light source there will be no shadows. Weird.
Addressing your questions one by one:
1. Would someone with Warlock's Devil's Sight be able to discern those lighting differences we consider important to our normal vision?
In dim light no. In total darkness yes, with the caveat that there would be no shadows. Darkvision is a separate ability.
2. Would someone with Warlock's Devil's Sight be able to detect where subtle shadows are when there's a faint light source to cast them?
No, it has no effect in dim light, only in darkness. Darkvision is a separate ability.
A devil's Devil's Sight works differently, however. For instance the Barbed Devil (MM p.70):
Magical darkness doesn't impede the devil's darkvision
So the answers for darkvision above would apply, with the caveat that magical darkness does not impede the darkvision.
Darkvision and Devil's Sight
3. Do the rules have anything to say, or is there guidance from closely connected material or other editions available?
The RAW from the 5e core rule books actually cover the questions you ask. I can find no other guidance specific to your questions in 5e supplementary material. As stated elsewhere, rules from other editions are not a reliable guide.
To list the sources defining darkvision that I have found in the core rule books:
- PHB p.183 Darkvision
- PHB race darkvision descriptions (e.g. p.20 dwarf)
- Monster Manual p.9 "monster" specific darkvision
- PHB p.108 Warlock's Devil's Sight
- Monster Manual p.70 Barbed Devil (and other devils) Devil's Sight
- (Sage Advice on Devil's Sight)[http://www.sageadvice.eu/2015/11/18/devils-sight-vision/]
And there are also rpg stack exchange questions on this, as referenced above:
- what is the correct interpretation of darkvision
- how should a warlock's Devil's Sight work
Best Answer
The Text is Clear
The Devil's Sight invocation reads simply:
Dim light is not mentioned, but is mentioned in every other form of Deprovision given to player features. This omission means that there is no benefit from the invocation until you get into actual darkness.
So, what "normally" means is that you can see if there were light; in full color, with no disadvantage imposed to attacks or sight based skill checks for reasons of darkness.
The Monster Manual definition of Devil Sight is specific to monsters with it, and don't apply to Warlocks, it doesn't change or affect how the invocation works.
Designer's Intent
Jeremy Crawford answered a question like this on Twitter in November 2015. while his tweets were still considered official rulings. Jeremy Crawford confirmed that Devil's Sight us meant to kick in only in total darkness. When asked:
He responded:
It is the designers intent that Devil's sight allows you to see in total darkness as if it were full light, but gain no bonuses in dim light. Other sources of darkvision would still apply.