"Nigger Tits" is an alternate curse word. It can be used as a variation of; shit, piss, damn it, fuck, or balls. Has nothing to do with a black person or her tits, however it isn't to be said around mixed company. It's usually said with an exclamatory inflection.
Person 1: Did you see that?!
Person 2: NIGGER TITS!!
Person 1: What's for Dinner?
Person 2: Liver...
Person 1: awww, NIGGER TITS!!
Person 1: Did you see the size of his chode?!
Person 2: Yeah, freakin NIGGER TITS!