>>96910642
>They only know that black is the evil color so only bad guys or former bad guys can have that.
June is literally monoblack as a card and she's not evil at all, she's a mercenary who's motivated by money but she's not a villain by any stretch and later teams up with zuko and iroh when they're "good guys".

The actual reason is that in the set's design Black doesn't have an associated Bending type, so it's less emphasized compared to firebending/waterbending/earthbending/airbending