When he was alive he had black and white skin, red eyes and wore a white suit and top hat. When he discovered he was dying of a "strange swamp fever" he decided to make one final record called Voodoo Scat and mixed it with a drop of his own blood.
In the episode "Electro" (Series 1, Episode 7) "Howling Jimmy Jefferson" makes his first appearance. His ghost is featured in this episode and referred to as the spirit of Jazz. In this form he looks the same as he did when he was alive but now has a number of super-natural powers and uses them to possess people such as Howard. Howard apparently made a deal with him years ago in which he would make Howard one of the greatest jazz musicians of all time. From that moment on whenever Howard played an instrument the spirit of Jazz would possess him and make him do "strange things" such as playing the trumpet so vigorously that his clothes tore off. The story is never fully resolved, however, subsequent episodes have shown Howard playing several instruments without any implication that he is possessed by the spirit of Jazz, so it can be assumed that the spirit has left Howard alone since.
Howard later obtains the voodoo scat record that Howling Jimmy made and Vince eats a piece of it which causes one of Howling Jimmy's blood cells (referred to as the jazz cell) to try and take over Vince's body. Howard shrinks himself down and enters Vince's body in an attempt to defeat the jazz cell, but ends up bringing it out of Vince's body with him and making it much larger. It is eventually defeated by a safety pin and walks away complaining. In the form of the Jazz cell Howling Jimmy looks like a ball of black dreadlocks with a head and top hat.