It's what inside the envelope that counts. The PO probably had a surplus, and they were told to get rid of them before selling any others. Hint: Most Black people don't care one way or the other about Black History Stamps. Like everyone else, they have their lives, families, jobs to think about. It is 'another portion' of the population that is pushing so-called equality. Commemorative stamps are not true equality.
True equality is this, we must all, Black or White, save our souls.
In the grand scheme of things, this is a trifle. The stamps will serve their purpose which is to pay postage. Go ahead and use them.