Jagz in Ascot (Station pub next to -guess what- yes, the station) has blues one day a week, I think it's Tuesday. South Hill park in Bracknell have a weekly blues night, possibly Wednesday (unless I've got them the wrong way round). 100 Club in Oxford St often has blues, Paul Jones Radio 2 programme often mentions upcoming gigs, try the BBC2 website. Mudcatters Steve Parkes and the_one_and_only_Dai are Bucks-based so may be able to help with your locality.
RtS (white boy lost in the blues)