There's clearly a fine line to be drawn here. If a song is in dialect, then singing it in a "received pronounciation" style sounds strange to me but a caricature accent (we all remember that lovable Cockney, Dick Van Dyke!) is as bad. Just as I lapse into my native Brummie when telling an Aynock and Ali story or slight brogue when telling my grandfather's Pat & Mike stories,and S or N Wales accents when teling tales from my sojourns in the Principality, I "sing" blues or '50s R 'n'R in Mid-Atlantic, sounding, I suspect, more Birmingham, England than Birmingham, Alabama, but would sound worse putting on an exaggerated "deep south" accent, or trying to sound like a 30s BBC wireless announcer. As long as we avoid sounding as if we are taking the piss out of the original accent, no one should be offended.
The way I sing, I offend all music lovers any way!
Roger [more Croakin' Bullfrog ("I got the greens!") than Howlin'Wolf]