I think all the other characters are fictional on his Earth. Pretty sure BL is not part of either the Arrowverse or Supergirl's world. They could still do a crossover by having him go to one of theirs or them going to his.

The Vixen reference in today's episode pretty much locks it in the Arrowverse. The way it was said implied Vixen is a person in their world, Vixen isn't known in the Supergirl universe. The Supergirl reference in this episode does as well, as you have to remember that Supergirl is known in the Arrowverse also because she and the rest were given public recognition by the Government after they stopped an alien invasion.

I guess it has to be Earth 1 then. I wish the writers would've just admitted it then instead of trying to be all like "It's not connected" BS.

It could just be the basic DC universe. Which means Black Lightning and his kids could end up in the Justice League like in the comics.