Iīm pretty sure that the African-American vote will make up about 30-35% of the total vote. There are about 5.8 Mio registered voters in NC, about 3.9 of them are Democrats and Independents. According to the NC Board of Elections, there are 1.2 Mio registered African-American voters. Letīs assume that 95% of all registered African-Americans are also Democrats and Independents, their share would make up 30% of all registered Dems+Indies. Not all Independents will turn out in the Democratic primary, therefore the share will most likely be higher, likely at around 35%.
PS: Poll cross-tabs
http://www.surveyusa.com/client/PollReport.aspx?g=d4d1bc7e-32d4-438e-8143-75c8b25b3435