Look punkfolkrocker!
In that 100 Greatest Blues Singer List ( The Blues Magazine, April 2016 ), only B B King and Etta james rank higher than Robert Plant! Please tell me you think that that is ridiculous!
And, Steve Marriott is rated higher than - wait for it - Aretha, Sister Rosetta Tharpe, Buddy Guy, Otis Rush, Leadbelly, Magic Sam. Mavis Staples, Otis Redding...
Now, you are entitled to like who you like, but I would be flabbergasted if I thought that you really believed that Stevie - and Robert - were greater blues singers than the legends listed above.
   Now, I know the list is a joke. For example, Son House and Skip James don't even appear in the top 100.
But, again, I will state that no Brit who sings in a phoney, pseudo-American accent should even be considered as a great blues singer. It wouldn't make sense!
   Now, it's fine for anyone getting up at their local folk club - like Will Fry said - and singing in any accent they like, but don't try to tell me that they are amongst the best singers in the world. That would be crazy.