If the backflush attachments don't work, rinse the black tank, completely.
Then, get to a dumping station that is on a concrete pad that slopes (like those at most rest areas on the interstates) shove your rinse hose up the outlet pipe to poke the blockage out of the way with the end of the hose (wear plastic gloves), until you can get in there and get it fixed.
Be SURE to flush completely, before you do so!
You don't need the bacteria contaminating the ground, thus the use of the concrete pad type of dumping station.
Rinse the concrete pad with Clorox when finished.
The Valtera twist-on valve is a good thing to have, in any case.