It's plainly relevant to show motive, intent, absence of mistake, etc. It's only inadmissible at a trial under rule 404 to the extent it is used to show bad character, but the rules of evidence don't apply at the grand jury phase, so we don't have to argue about rule 404. It's relevant and admissible.
So your basic position, if I understand it, is that if a white person is not indicted on a charge but a black person is indicted in that same charge, albeit for separate events that happened in separate places and weeks apart, that is prima facie evidence of discrimination, regardless of the underlying facts of the charge?
You sound like the "eyewitness" in Ferguson who testified that he knew Wilson shot Brown in the back because of where he was from.