Suppose a Sorcerer with Subtle Spell is blindfolded, and has their hands tied behind their back. Could they Subtle cast telekinesis to remove the blindfold?
The relevant part of the spell description:
When you cast the spell, and as your action each round for the duration, you can exert your will on one creature or object that you can see within range, causing the appropriate effect below.
In short, do they see the blindfold when they open their eyes (even if the blindfold is pitch black)?
The definition of "see" means you can visualise the target or include it in at least one of your sight senses; as the only thing blocking your normal sight is the blindfold, the blindfold is currently in your vision.
However, going by the rules as written, there's a hilarious wrinkle:
If the object is worn or carried by a creature, you must make an ability check with your spellcasting ability contested by that creature's Strength check. If you succeed, you pull the object away from that creature and can move it up to 30 feet in any direction but not beyond the range of this spell.
As there is no listed exception for objects being worn by the person casting Telekinesis, you'd have to contest your own Strength check against your own spellcasting ability to be able to successfully remove the blindfold from yourself.
Of course, this is pretty ludicrous and I imagine most GMs would choose not to enforce that unless you'd really annoyed them that day. :P
The problem here is that you have been blinded. I am making an assumption (but will update if the question clarifies) that the character is now blind based on the tag included in the question itself.
If they are blind, they can not see. It's not that they only see the blindfold, but that they cannot see at all. If they can't see, then they don't meet the spell's requirement.
with a note below.
The requirement for exerting your will is that you can see the target, that is the blindfold. Unless you are in darkness, you would be able to see the blindfold. The amount of light needed to see the blindfold is open to the DM's ruling. As a general rule, you would be able to see the blindfold if at least the blindfold is within dim light area.
The blindfold being worn is actually not a concern, because you won't resist.
Note: Stricter (and ridiculous) reading would require you to still perform the contest. The DM may grant advantage to spellcasting ability roll and disadvantage for strength roll for not resisting, though.
If the blindfold is secured with a knot, more realistic DM might consider whether you can't see the knot. Most of the times you can't see the knot, thus won't be able to untie the blindfold. You might still be able to remove the blindfold, though, if it's not tightly secured.
Note: Stricter reading would disregard the position of knot. You can see the blindfold, so you can manipulate the knot wherever it is.
So, I would ask myself these questions:
