A monolith is a large, unified structure, like a skyscraper or a big block of concrete. To say a group is monolithic implies they are all alike, that they act as one in accordance to their structure (or race): "a large and impersonal political, corporate, or social structure regarded as intractably indivisible and uniform."
To say a group is monolithic sounds condescending; if it said by a white man it sounds racist. Regardless, it seems stereotypical. Obviously, it should be avoided.
I assume the context here is sociological. African-Americans were originally forced immigrants, moved to the Americas during the Diaspora that was the slave trade. In this sense, they were taken from diverse places and not given an education, rights, or representation when they were transplanted here. In this way, they were not a unified, culturally-same people. But then, who is? I don't know any culture, race, or ethnicity that I would classify as monolithic (it's a limiting, formless word). Maybe some isolated island colony?
The great poet Derek Walcott, who was a half-white and half-black islander, said, "Either I'm nobody or a nation." He would not say either of his races were monolithic; rather, he (as an individual) is made up of both the white and black culture, the island culture, the American culture, the English language, etc... All these are assimilated in one man. Otherwise, he is a nobody.