You have to remember that that particular scene takes place in the 1880's, and in the 1880's, blacks were not allowed to perform onstage, especially in the South. So every time a black character was portrayed onstage, he/she was played by a white actor with blackface makeup. If you had been able to see a performance on a show boat in which a black character appeared, you would have seen a white actor with black makeup.
We don't know what the portrayals of blacks were actually like on 19th century stages, but I'd guess they were similar to what was done in that song. They probably were pretty exaggerated. Minstrel shows existed back then.
The only black character portrayed in a dignified way onstage back then was probably Shakespeare's "Othello", and yes, in the 19th century, Othello was always played by a white actor with black makeup.
Of course, that blackface number in "Show Boat" wasn't really necessary - it's not from the original show. It was written by Kern and Hammerstein especially for the film.
In 1936, Al Jolson was still doing blackface for a living, Mickey and Judy were doing blackface at MGM, and it was still viewed as a valid, if more and more unsettling, form of entertainment. "Gallavantin' Around" can be excused in a film where the southern black characters are treated with dignity, respect and authenticity, if not necessarily with the "political correctness" of seventy years' worth of hindsight. In the stage show, the spot was filled with an "olio" dance for Frank, which would not be too persuasive in an Irene Dunne film. Besides, the audience isn't laughing at the character. They're laughing at the prop bird that gets stuck "mid-flight" during the number.
That's a very misleading statement that you make about Jolson actually, he was not "doing blackface for a living" in the mid or late 1930s but had already moved beyond that part of his act for the most part. The only time I've seen him do blackface around that time was in the film version of George Gershwin's biography, "Rhapsody in Blue," in which he was supposed to be portraying himself in the early 1920s singing "Swanee." Furthermore Al Jolson was never a traditional minstrel performer except perhaps in his very early days. A lot of what he did was to try to undermine the stereotypes through the use of blackface. But it all kind of falls under a generic category in people's minds these days.
Did I not love him, Cooch? MY OWN FLESH I DIDN'T LOVE BETTER!!! But he had to say 'Nooooooooo'
It was unessacary to the plot of the movie. And thanks for clearing that audience laughing scene up for me.
I buried those cockaroaches Tony Montana (Scarface)
It would be difficult to have a musical film called SHOW BOAT and not have a musical number exemplifying that particular type of entertainment. "The Parson's Bride" scene might suffice for a non-musical film, but not for a musical. You need a "show boat" performance number somewhere. It's also dramatically sound to have a "performance" number for the character of Magnolia early in the proceedings to give a hint of her future success as a performer. In the play, the olio number was given to Frank to cover a costume and scenery change, unnecessary in a film.
And not all songs have to move the plot along. Certainly "Ol' Man River", wonderful song that it is, doesn't move the plot along one inch. Sets a mood? Yes, wonderfully. Advances the plot? No, not a bit. Nor do all show songs have to.