In a bit of a predicament (Black Water tank full and frozen)

If it's just a build up of solids from neglect, get the proper holding tank "chemical" and use it exclusively. The blue stuff just is a deodorant. Get an enzyme treatment. It will take a few fill/drain cycles to get fully working as the blue stuff inhibits it from working properly so you have to flush all remnants of that out of the tank to have full effect. Once you do though, it works great. It digests waste and toilet paper and makes everything come out when you drain. It also prevents odors.

EXAMPLE OF WHAT I MEAN