Oh, I forgot to do a wash on the canvas before started, to knock back that stark white to some middle tone, so better late than never. Even stranger was the thinned wash I used beaded up a bit on the surface not immediately "bonding" with the gesso. Not sure why. Maybe it was something that happened in storage.
After a bit of scrubbing I got it to mesh in nicely, and naturally it mushed the charcoal about, but that's fine, since it helped prevent the charcoal from meddling with successive applications of color.