We have the same type of problem but on the gray tank side. Sounds like it is a disk from the hole saw used when the tank and piping were installed. I would have to drop the tank to get it repaired and that's way too much money. Found a solution that works for us though. Buy a third valve that attaches to the outlet where you connect your sewer hose. Close the black, grey and third valves and fill your gray tank with water to the very top. With all three valves closed there will be a air bubble trapped in the the piping that is between the black, gray and third valves. When the gray tank is full of water, and keeping the third valve closed, open your black tank valve then open your gray tank valve fully. The weight of the water will force the air bubble back into the black tank and "blast" the black tank obstruction out of the way. Now close your gray valve and open the third valve and your troubles should flush away. Let us know how it goes.
