During the pre-Civil War era, Africans could not even be referred to as African-Americans, because they were considered property. They were "uprooted" from their native homes (Africa, mostly) and "transplanted" into a foreign country where everything was different - weather, customs, food, people. They were removed from their families and thrown in together with other people that they did not know - new family, new world. They were therefore removed from the cultural group from which they obtained their "ethnicity." The first of the two concepts were the strongest during this period because I don't believe they even thought about their eroding "ethnicity" - they were in survival mode. Even when they had children, their families were separated, so the cycle of uprootedness and transplantation continued, although now, the transplantation was not as pronounced since they were not being moved to another country, but to another place within the same country.
During the Civil War and Reconstruction, transplantation and uprootedness continued but again, it was inter-country, so maybe not as pronounced as when they were torn away from their native countries. During this period, they perhaps began to focus on their ethnicity more. Who were they really? They were Africans, but by this time, most of them had not been born in Africa. They were living in America, but they were not really like other Americans - they were downtrodden, they were not treated equally. They were living in Jim Crow America, even though they were emancipated. Most could not vote and persecution and prejudice were rampant. This was when the KKK arose, after the Civil War and during Reconstruction. I think that during post-Reconstruction, there was the strongest concentration on dual construction of ethnicity because the people wanted to maintain their African culture, but they also wanted to be accepted as free Americans -- African-Americans.
In some ways, African-Americans are still struggling with a dual construction of ethnicity. For example, a personal friend of mine recently chastised her daughter for wanting to name her child (my friend's grandaughter) Abigail. My friend told her daughter that this name was "too Eurocentric" and that she should pick something more along the lines of their ethnicity. So, the little girl's middle name is Abigail.