Maybe not on the national level, but preventing their decline into violent radicalism and eventually criminality would be helpful. I think your best bet is to prevent the assassination of Fred Hampton Sr. in Chicago. If he, a moderate who was known for his work across racial lines in developing the leftist "Rainbow Coalition", manages to survive and eventually gain control of the party, the BPP might have a chance of remaining a major player in the politics of certain urban areas. That might lead them to unite with other, non-black far-left groups and become a faction of the Democratic Party (or even mount a real third party bid from the Left).