why is there a whole separate black greek system?

They're historically black fraternities that were founded at HBCUs (historically black colleges and universities). They don't "rush" so much as recruit. Their pledge classes tend to be smaller as well. What I can say (bc I'm not in one so I don't know the whole deal) is they're v lit and the guys that I've met from the black frats are extremely classy/know how to have a good time. The only difference I really see is they tend to mix with the historically black sororities like Delta Sig, etc. while the "normal" frats mix with Panhell sororities, ig zeta, chio, DG, etc. what you choose depends on the experience you want. Im guessing the IFC frats like Sig Chi have super expensive dues compared to the black frats since they have housing.