I also felt that Mitchell's number when he blackens only half his face was a good idea that didn't work that well in context. It evoked all sorts of potentially interesting relationships, the Janus masks of tragedy and comedy, the character's refined, real self vs. the falsified stereotype he portrayed on stage, etc. However, even as I was watching it, it caused my mind to wander back to the more memorable and effective Solomon Song done by Kevin Kline in the Wolfe-directed Mother Courage. Assuming my memory is correct (it may not be), both numbers appear late in the show, both involve the applying of make-up, both are delivered bitterly and ironically, and both even feature similar angular hand and arm movements.
One big difference is that, prior to the point where the song appears, the book for Shuffle Along has been superficial and cliched. There's an attempt to evoke deep feeling in regard to characters and situations that are barely sketched, and to hang weighty meaning on an exceptionally slight frame.
