The word 'blues', like the word 'folk', is open to interpretation.

To some - let's say the purists - blues singers represent a downtrodden and repressed segment of American society, and their music is more than music. It has social and racial significance beyond the music which cannot be, by its very nature, be sung authentically by people other than black Americans.

To others, 'blues' is simply another muical style or formula, open to anyone to try their hand (or voice) at, like jazz or opera or rock'n roll. In this sense, a singer's origins are irrelevant, the performance being more important.

As to 'good' and 'bad', or 'best' and 'worst' - it's a matter of individual opinion in the end, as all opinions are individual and subjective.

For myself, I used to enjoy American blues singers when I was in my teens and into my 30s, until other music began to interest me - so I have no opinion either way.