As of 2007, membership was estimated at around 50,000.
8,800,000 african americans in the USA.
So, that’s about 0.6% of black population, assuming that members believe everything of their group.
At the same time, even among non-members, there’s likely to be some influence in some black communities, so it’s hard to say.
But as it’s likely that a majority of african americans are NOT members of the Nation of Islam, there’s a very good opportunity to reach out to african american youth in STEM fields, as most will not be influenced by an ideology that includes an anti-Science figure.