It sounds like the vent stack is plugged.
DIY or have somebody else go up on your roof and look into your vent w/a flashlight. If there's something in there, try a wire clothes hangar if it's close enough to the top. If it looks like it's big/solid enough to plug your sewer line if pushed down into your stack, call a drain cleaning company. They can put a snake down it and pull it out.
As for the black crud, it's actually build-up from normal use. I clean it out of customer's pipes all the time. It's soap/grease/human grease build-up. Nasty as it is, using drain cleaners with acid in them to get rid of it will damage metal pipes. Look in your local hardware store for a drain cleaner with bacteria/enzymes. They eat the nasty stuff and won't harm your pipes. Some drain cleaning companies carry it - it's a "drain maintainer" or for preventive maintenance. If you go that route, ask for a free sample or a demo before buying.
"And away go troubles down the drain." tm