Several blues players "de-tune" a semi-tone. Some claim it's because they want to use the heaviest gauge strings they can, for tone, but still want to be able to get some bend on the string, and others, because it makes life easier if you're playing with a horn section(all those sharps and flats).
I know Stevie Ray Vaughan fell into the former group, whereas I, who hated playing the blues in E^b, the latter.



Me and SRV in the SAME sentence? .....Hmmmmm!