because its too easy to sabotage. districts can be redrawn, the main parties can field candidates to cause competition, etc. out of all the congressional districts in the US, which are majority black and which could be won from Democrats?
there are 435 congressional districts, 44 of which have black representatives or are majority black. 26 of that 44 is majority black (50% or more). All of them are Democrats. There's no guarantee Republicans with all their resources could win those seats let alone a new "black party".
http://en.wikipedia.org/wiki/List_o...ed_States_congressional_districts#cite_note-1
a better idea is to start local, instead of at the very top. Consolidate municipal power in black populated areas. then work to county then state