Why?
If it's a mere case of not wanting it diluted, then that needn't be an issue. As stated above, Black Lightning is very much your Daredevil or Spider-Man in that he's only in operation within a very small community; possibly one that neither Kara or Clark have even heard of. I suppose there'd be some issue with aliens existing (and the President being one) if this was set on Earth-38, but equally, it could be set on yet another Earth with a different Clark and Kara; one where those two are the only aliens on Earth?
Is it that you'd rather this show not be on Earth-1 or Earth-38, or that you'd rather this show not remotely be connected to
the Arrowverse in any way at all? I can understand the desire to have it remain on it's own turf, because it's a very different show to the other CW shows, but I think it would be a little short sighted of the writers to be too far removed. I want this show to be within the multiverse, I just don't particularly want Pierce crossing over with Earth 1's Barry, or Earth-38's Kara anytime soon.
If anything, and as I've said before, the only Arrowverse character I could see on Black Lightning is Arrow because of the similar tone. Flash & Supergirl are too ...
light.