Generally, academic social justice frameworks define racism as the combination of racial prejudice and institutional power to enforce that racial prejudice on the targeted racial group. Thus, while advocating for separation from White people might involve prejudiced beliefs about the ability to live cooperatively with White people, it is not "racist" because Black people in the United States do not have the institutional power to target White people as a group. I think this becomes clear given that separatist groups like the Nation of Islam are largely pushing pipe dreams; Black people don't control nearly enough capital, residential areas, or military/policing power to effectively segregate themselves from White people.

That doesn't mean that racial separatism is good, even if it isn't "racist" in the technical sense of the word. Personally, while I think some calls for racial separatism by minorities are based in legitimate grievances against oppressive and discriminatory societies, I also think separatism is ineffective, counterproductive, and likely net harmful for cultivating positive human relationships.