Perhaps misfiring, and those are fuel deposits? Or perhaps a leaky valve seal?
Another modern "improvement" that crab cap. I had GM cars with both the old points ignition (1972 Chevy Impala 350) and the later HEI system (1975 Olds Delta 350) and neither one had problems with the standard Delco caps. Same with my old boat, I find that I usually get 5-6 seasons from a set of points, whereas in the old days we used to change them yearly as a matter of course.
I actually prefer stone age systems, points and carbs, I can diagnose and fix them easily, and no electronic mystery boxes, chasing bad connections, etc.