You're right about the "sidekick" angle. That is the role he played in many films. It is the fact that Sam was not only black, it was a part of his personality in the film.

An odd part of the story is that in real life, Hoagy cast himself as a piano-playing best friend. When a young man, Hoagy and Bix Beiderbecke were inseperable companions. After Beiderbecke's unexpected death, Hoagy remained Bix's friend, helping to keep his memory alive. He even named one of his sones after the cornet player. In his home, he had on display, the mouthpiece from Bix's horn.

(I guess this isn't a story, just some memories shaken loose by Parothdone.)

Sourdough