A few would say it was not necessary. That would make it very necessary. So many shows are afraid. The only other one on the CW that I would kind of sort of expect it to happen, maybe, would be Arrow. The funniest thing about the moment, for me (as a black man), is that the second it was said I literally stated out loud "Well I'm guessing this means Supergirl won't be guest appearing on Black Lightning anytime soon. Realistically, the only thing I can see happening first a visit from Jimmy Olsen to have another one of those "Sometimes they'll only see me as a threat" episodes. This may clear the way for a possible Supergirl cameo but in reality, even seeing a black version of Jimmy would still feel out of place.
Maybe Sisco? His dark past and delivery can easily be made to fit into place with the tone of Blacklightling. Sisco and Jennifer would be a very comical pairing. Then again,
The only thing now that makes sense is a cameo by David Ramsey as John Diggle. After the gauntlet has been drawn, this would be a winning cameo that can bring Black Lighting into the team. To me, its the only logical anchor to the rest of the shows. Wild Dog and Mr. Terrific could also make a splash. Just my two cents.