First time owner-Black water tank clogged....help!

Personally I would close the black tank valve & add a good healthy dose of Rix-x and let it sit 24 hours then add some more water and drain. Repeat this a couple of times till tank drains well. After that I would use with the tank valve closed and and add a dose of Rid-x after every empting for a few cycles to eat up all the solid matter in ther. After that only open the black tank to dump.