Wipe that smile off your face. Blues songs are about pain, death, losing your boyfriend or girlfriend, cheating or being cheated on. Sound hurt, but don't whine. Blues songs are about feeling too deeply, not self-pity.
Work your lower register. Most blues songs are meant to be sung in a low or medium pitch. Higher pitched voices are too spry and chirpy for songs like Leadbelly's "Where Did You Sleep Last Night?"
Learn to sing with a slide guitar or harmonica in the background. Blues instrumentation is slow and sensuous, often with a shuffle rhythm.
Shed your inhibitions. You'll sing about shooting your lover in a barroom, sneaking off with a married lover, being broke and drinking cheap wine. You have to sing the blues like you mean it. There's no faking it while singing "Little Red Rooster" or any Willie Dixon song.
Adapt a blues moniker before your first performance. No one will buy the blues from a Tiffany or Jason. Try names like Wille Jo or Big Sadie.