Read the label....RidX is for septic tanks...that is..tanks where poop sits for awhile and the bacteria action of RidX can go to work. Your black tank if for TEMPORARY storage of poop before the tank get flushed out.
IMHO, clean your black tank completely using the Geo method or whatever you prefer, then flush well. Use ample water when flushing and ample water when dumping and you won't need all the extra junk in an attempt to make it smell good. In over 50 years of camping, I've used nothing but water in my black tanks and never had any smell....not to say my )(*& don't stink....:B....just sayin....Dennis