If the enemy mage casts a spell and I describe to everybody that it becomes pitch black all around them, they know that a Darkness spell is going on. If on the other hand I ask for a CON save and (after they failed) tell them that they can’t see anymore, they know the mage cast Blindness on them.
Similarly, if it becomes dead silent in the area, they would assume that someone dropped a Silence spell. But if I describe that they can’t hear anymore (after failing a save), they would think that they are now under the influence of a Deafness spell.
But how would they know the difference? Because at first glance I would imagine that somebody inside the effective range of a Darkness or Silence spell cannot tell if a spell messes with their head or with the environment. Or is it obvious what happens when the spell takes effect? (For example, a Darkness spell expands from the center or gradually makes everything darker, while a Blindness spell just kills your sight from one moment to the next?)
Because although losing sight/hearing due to Blindness/Deafness compared to Darkness/Silence seems identical from the character’s point of view, they have completely difference options in both cases.
A deafened spellcaster can cast spells with verbal components, but a spellcaster affected by Silence cannot. They could try to run out of the effective range of the Darkness/Silence spell, but they cannot run away from Blindness/Deafness. A spellcaster could cast Continual Flame at 3rd level to negate the Darkness spell, but this wouldn’t help if he/she is blinded.
At the same time they would have to make saving throws every turn and maybe they notice that (somehow)? Then again, they are in the heat of battle and could be too distracted to pay attention to the minute differences between the various options of not being able to see/hear.
Is this something I can use as a GM to mess with my players every now and then? Or would the players feel hassled, because the differences are obvious?
Best Answer
Standing still, there is no way to tell via mundane means
Darkness vs Blindness
Being inside an area of darkness puts you inside a heavily obscured area. This is true whether the darkness is nonmagical or magical (as in the one created by Darkness).
All creatures inside a heavily obscured area suffer from the Blinded condition.
Silence vs Deafened
An area covered by the Silence spell makes a much more direct comparison to Deafness:
And so, there is no way to tell.
Except...
You can leave the area of effect
Both Darkness and Silence create static, non-mobile areas of effect. An easy, mundane way to know is to walk in a straight line until you have walked a certain distance. If you are still blind, it's the Blindness spell; otherwise you will be able to see the area of darkness behind you. If you are still deaf, it's the Deafness spell; otherwise you will start being able to hear again.
Alternatively, you can cast Detect Magic
Blindness/Deafness is a necromancy spell, whereas Silence is an illusion spell, and Darkness is an evocation spell. If you could cast Detect Magic, you would be able to tell the difference between these schools.
You can also try casting spells
Spells which require a Verbal component fail entirely inside an area of Silence, but you can still cast them while Deafened. For example, you can use a Healing Word on yourself. If you regenerate hitpoints, then you must be under the Deafness spell.
Can you use Blindness/Deafness/Darkness/Silence as a tool?
On your final question: is it a tool you have as a DM? Yes, of course it is. You could set up traps and puzzles that require sight or hearing to be gone. Or you can have deaf party members due to the Deafness spell trying to communicate with the other non-deaf party members. It's fun (for you, at least).
Will the players feel hassled? That depends on them. If you would really like to spring it on them, I say give it a shot. But if you genuinely believe they will strongly hate it, ask them to be sure. The reactions of your players will depend on that particular set of players and is not something the people at this Stack can determine for you.
Your job is to present them with obstacles and puzzles to challenge them. This is just one of those ways to do that. So yes, it is within your toolbox if you want to use them.