Yes, darkness does block vision.

There's a lot of answers to this question already, supporting both sides, but this seems like a good place to put the only "official" answer I was able to find, a tweet from Dan Dillion, here:

Is the area of a darkness spell opaque, i.e. blocks line of sight? So, can two creatures in a large well lit room standing on opposite sides of the area of a darkness spell see each other? I’m guessing not but just trying to reconcile the spell against how normal darkness works.

Without some ability to see through darkness, whether magical or mundane, that area is impenetrable to vision (heavily obscured, in game terms).

Yes, darkness does block vision.

There's a lot of answers to this question already, supporting both sides, but this seems like a good place to put the only "official" answer I was able to find, a tweet from Dan Dillion, here:

Is the area of a darkness spell opaque, i.e. blocks line of sight? So, can two creatures in a large well lit room standing on opposite sides of the area of a darkness spell see each other? I’m guessing not but just trying to reconcile the spell against how normal darkness works.

Without some ability to see through darkness, whether magical or mundane, that area is impenetrable to vision (heavily obscured, in game terms).