I stopped in at Dyno and was told that the material was "negative plate expander" and was not really a problem, just a normal sign of battery aging. If the batteries were overcharged, it could be "excessive," although the ultimate indication of overcharging would be early battery failure. So maintaining the water level, use a hygrometer, maybe stress test the battery, watch the charge cycle, monitor, etc. are all more important than the black goop.
I thought that lead oxide was alway red, as with the red lead oxide
paint I used on wood boats. Turns out that there is a black lead oxide used in battery production as shown in the final entry of this Wikipedia post
. One thing less
to worry about.