"Nasty" black stuff? Sorry for maybe being ignorant, I am new to cast iron cookware, but I thought the black stuff is what we appreciate and cure the pans for?
The smooth black parts, yes. The gunky/bumpy parts on the handle, no. That comes from years of continued use and improper cleaning where new oil polymerizes over leftover particulate the next time it’s cooked on.
But I wouldn’t recommend a dishwasher cycle as the method to use to strip your pan either. Self cleaning cycle on the oven should take care of it.