If treated appropriately, Black ich is much less a concern than C. irritans and much easier to get rid of. They are not the same type of parasite either. Crypt is a ciliated protozoan while Black ich (Paravortex) is a flatworm. Black ich will not usually kill the fish anywhere near as quickly as crypt would. The exception being where water quality/husbandry was poor and allowed for the easy onset of secondary infection.
If a FW dip does not show good results on the first dip, formalin would be the next step. Barring that, organophosphates would be the final resort but should never (or at least rarely) be initially considered. I would start with the FW dip in almost all cases where Paravortex was suspected. Any other external worm or copepod I would go straight for formalin.
None of these options would work for C. irritans.
Cheers
Steve