Since when is Robert Alexander "Bumps" Blackwell an authority on what to call Blues?
You're right, he might have discovered Little Richard and co-written some of his hits, but then Little Richard is - like Keith Emerson really - already too far removed from the blues to count. And Sam Cooke, but that guy had nothing to do with blues either, because blues and soul are really like man and ape, not related at all.
The only authority in the blues apartheid state is of course Nofi, grand wizard of definition who is in and who is out.
"DP is not a blues band."
Thanksfully not, but I'm not aware that anybody made that claim. But they could play a blues tune unless your definition of blues is that it cannot be played by anyone who plays nothing else but blues. How elitist.