Seems like I get this question all the time. Here's a few thoughts to help you recognize a real blues band:
I think there's a difference between a good blues SONG and a good blues BAND. The best blues bands I can think of have a few characteristics in common:
1. The gig is an on-going conversation where the audience is a key participant. There's no rock-n-roll 'we're too cool to acknowlege the little people trip'
2. Song selection is geared for dancing, dominated by medium-tempo grooves that allow people to dance by shuffling feet and moving pelvises. Slow songs, funky songs too. Anything resembling 'Freebird', 'Enter Sandman' or 'Takin care of business' is a give-away that you're not listening to a blues band.
3. Likewise, songs that are not about collective concerns of a room full of people that work for a living generally aren't blues songs.
4. If someone in the band shot a man in menphis just to watch him die, that's a country band. Learn to line dance when listening to this band.
5. But, a band singing "i'm going to murder my baby" probably is a blues band, but only if the tempo of the song is one you could also use to walk to the dentist.
6. A band full of people dressed in shades, fedora's and cheap black suits is not a blues band, it's a movie tribute band. Run, don't walk, in the opposite direction.
7. Some indicators that you've found a real blues band- they can handle a Magic Sam request, have day jobs working in a steel mill, are not playing shiny guitars. If something about the singing or the guitar playing sends a shiver down your spine, makes you want to shuffle your feet and every once in a while turn to your date and smile because you've heard something universal about the human condition (excluding BMW cars, hired help, tax shelters and a CEO sucession plan) then you're listening to a blues band. If you are listening to songs about BMW cars, hired help, tax shelters and a CEO sucession plan sung by men in suits, fedoras and sunglasses, refer to #6 above. Should you by chance encounter a real working blues band, throw a fiver in the tip jar, buy a CD or two and tell them to keep up the good work. There's too many #6 bands around.
Are we clear now?