The Blues, like other musical genres, is flexible. It expands and contracts with the passage of time, and changes with the milieu in which it evolves. Mozart was considered revolutionary in his day (he put 'too many notes' into his music); jazz expands to cover hybrids like 'fusion' in which jazz greats like Miles Davis were instrumental. Country music absorbs the influences of Hank Williams and Chet Atkins, to become something else markedly different from what it started out to be, yet it can still be classified as 'country.'

The Blues is expansive enough to include guys like Hendrix, Clapton, and Page when they're paying tribute to the influences that spawned their styles. It all boils down to how narrow one wants to define his/her parameters.