I'd say that the 7* version of Black Cat is pretty useless on defense, yes, but I've actually gotten quite a few KO's with my 6* Black Cat when I've placed her on BG defense. At max sig, she's a nightmare to intercept. That can trip people up in general fights, but in fights where you explicitly have to intercept her for one reason or another, she's a menace.But that tactic doesn't work with a 7* since you ideally want her at max (or at least high) sig for that. Edit: but Chee is still the better defensive option, obviously.
Bishop and Chee'ilth, close one with Black Cat but personally I prefer Chee'ilth cause Black Cat is useless on defense.