If they ripped off riffs from some of those local artists they'd be as culpable as, what, half the popular musicians producing music in the late 50, early 60s?
It’s one thing to cover ;killin’ floor’ and credit Howlin Wolf, it’s something else to change a few lyrics, call it ‘The Lemon Song’ and claim you wrote it.
ZZ Top has covered many Willie Dixon songs, but the credit him, Zep took ‘you Need love’ changed it a bit and called it their own.
Not to detract from Zep, or the fact that it was the great English musicians of the 60’s that FINALLY brought the attention and renumeration so long due to the American Delta and Chicago blues musicians.