When it comes to pinpointing your DNA, most Americans of enslaved African ancestry come from only 46 ethnic groups.
The areas that Africans were enslaved in and brought to America from during the transatlantic slave trade were few and were documented by both European and American slavers.
This article explains in detail how DNA is used to find out where your African ancestry is traced, both by country and by ethnicity.
When tracing my DNA, I found out that I am 32% Nigerian and 19% Malian which is common for people who are descended from Nigeria. While Nigerians certainly see themselves as citizens of Nigeria, they recognize their ethnicity first. I will need to get further DNA testing done to determine which of the 300 ethnicities in Nigeria I am genetically connected to.
I recommend that you take either a maternal (if you are female) or paternal (if you are male) DNA test from AfricanAncestry.com to get the full picture of both countries and ethnicities.