Certain spells require the target to be able to see the caster. Is a creature that uses blindsight considered able to "see" the caster? What about if they're seeing the caster via darkvision only, or truesight only, etc?
For example would a ranger be able to cast animal friendship on a purple worm, which only has blindsight & tremorsense, given that the spell requires the target creature to be able to "see and hear" you?
(I'm assuming that tremorsense is a vibration/sound ability, not sight.)
Truesight is what it says on the box: Sight. Specifically,
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects...
Similarly, Darkvision is vision, and is specifically stated to be seeing:
A monster with darkvision can see in the dark within a specific radius.
So there's no question about those two. If you can see something with truesight or darkvision, you're seeing it.
Blindsight is specifically not seeing:
A monster with blindsight can perceive its surroundings without relying on sight
The "Vision and Light" section of the rules (PHB p.183) repeats this information, and seems to be indicating that blindsight means you can ignore things like darkness and obscurement, but it isn't clear on that point.
However, the Sage Advice Compendium comes in to give us a specific, official answer to this:
Can a blinded creature make an opportunity attack?
An opportunity attack is triggered by “a hostile creature you can see” (PH, 195). If you can’t see an enemy, you can’t make an opportunity attack against it. Creatures with blindsight are an exception to this rule, because that ability lets those creatures “see” within a certain radius.
So officially, blindsight counts as seeing for the purpose of abilities that depend on seeing an enemy. If it's good enough to count as seeing for opportunity attacks, it should also count as seeing for targeting a spell.
That said, this is going to require some DM guidance. Does blindsight make you more vulnerable to a medusa's gaze, which depends on whether you "can see the medusa's eyes"? It seems obvious that blindsight shouldn't count as sight for that purpose even if it counts as seeing for some other cases; it would be absurd to suggest that a bat's echolocation could cause it to be turned to stone! So, the DM will have to make some determinations. I would suggest that the DM should generally rule in favor of the creature with enhanced senses (that is to say, blindsight should count as seeing or not-seeing, whichever is more advantageous to the creature with blindsight in the situation at hand), but it's going to be somewhat ad-hoc, and that's okay. That's what the DM is there for.
Tremorsense is described as:
A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius...
"Detect and pinpoint" is an interesting term to use. It sounds like tremorsense gives the creature your location, but not actual sight-equivalent (and thus would not, on its own, allow for attacks of opportunity or spellcasting that requires seeing).
Further evidence comes from the fact that Blindsight specifically notes
If a monster is naturally blind, it has a parenthetical note to this effect, indicating that the radius of its blindsight defines the maximum range of its perception.
Tremorsense carries no such note, which suggests that tremorsense does not count as "perception" for an otherwise blind creature (for example, the purple worm, which has a short blindsight range and a much larger area of tremorsense).
Based on the evidence, I'd say Tremorsense tells you where something is, but not what it is or what it's doing. It is, perhaps, more analogous to hearing than to sight.
A 14th level Rogue has Blindsense, which specifies:
you are aware of the location of any hidden or invisible creature within 10 feet of you.
Clearly this is not sight; it's merely location-awareness. You're hard to hide from, but you still can't use it to make opportunity attacks against things you can't see.
The rules are here.
Vision, Darkvision & Truesight are all forms of “seeing”.
Blindsight works “without relying on sight.” This can be an unknown sense beyond the standard 5 or something like a bats echolocation depending on the monster.
“Tremorsense can detect and pinpoint the origin of vibrations” so it’s touch based.
