Thank you. I was wishing afterwards that I had of added to my "O'Clock Jump" comments of how Bix' music is described as happy and zestful but at the same blended in with an air of melancholy. Here is Herschel blending together spirited jubilation, but with a distinct air of melancholy or sadness throughout. There's a connection and an artistry that few shared.
I'll have to think more on "Singin". It's overall impression to me is that it's a mood piece, subdued and thoughtful, and as such is quite likely the father of the Jazz ballad. I wouldn't use the definition of moody in its 19th century literary sense. In a jazz context I think of it meaning subdued or funky, oh I see brooding, sombre, that's good. I'd say that. If you don't get impressions of them from "Singin", compare it to that other father of the jazz ballad, Hawkins on "One Hour". Compared to "Singin", "One Hour" just seems like a bunch of notes played slowly.
Maybe some other impressions of what "Singin" is and represents will come in.