Blackriver’s outfit is Anura designed, and its transmutational properties require her genetic signature to work. At puberty these catgirls lose the ability to shift forms at will, and become either more girl or more cat for their adult lives.
Blackriver’s outfit, designed for her by Anurae (AHP.7C4), circumvents her feline form. Nellko’s outfit has some enhancing properties as well, though she does not require it to remain humanoid. She just likes her outfit.