Some bluesmen have been known to keep a sartorial style all their own (SRV comes to mind with that hat of his). And a few serious jazzmen have been serious fashionistas (Miles Davis, Roy Haynes). So it's not always "all about the music." As always, a case to case basis.
For any musician though, it's usually encouraged to look like a musician on stage, not look like you just drove to the gig straight from the office. 
There's a misconception with the old black bluesmen, in particular, that they liked looking the part of the "poor, old hobos". Well, those dudes bought nice guitars, suits, and cars as soon as they can afford it, especially after they wax a couple of 78s uptown! But more often than not, they were poor and it showed in their gear and getup. Their consolation was the occasional promotional studio photo with the borrowed suit and guitar.
Muddy Waters came to Europe wearing fine suits that demonstrated his stature in bluesdom. With his electric band, he wasn't always well received. He came back the next year in overalls plus acoustic guitar and got standing ovations everywhere.
