We can argue this by saying that such colleges and universities give African Americans the ability to thrive in an environment that is less threatening than the environment of a regular college or university.
At HBCUs, African Americans are in a relatively familiar environment. They do not have to worry about what white classmates think of them. They do not have to worry that their white classmates will think that they are in the school only due to affirmative action. Because of this, African American students might do better in such schools.
Later, when they are fully-educated and more confident in themselves, they can prosper in integrated society. Until then, HBCUs help them gain the education and confidence they need in a relatively safe environment. This helps to improve society.