Currently experiment antiblack with an immunity to having your creatures power and toughness be lowered (to protect again -?/-? effects that only black has access to, also stops power being lowered so feels more red as toughness alone would feel more blue-white-green that four coloured). I have no ideas if this works within the rules.

Maybe change it to "cannot be lowered by spells or abilities your opponents control" to avoid crashing the rules.

If it doesn't work just give hexproof variant that only protects from mono-black.

Hexproof from black is now a thing. Though I would have gone for straight hexproof.