under the advanced search there is an entry blank for "birthplace" leavr the names blank, and fill in state (and county) and fill in birthplace Africa

In the 1870 and 1880 you can still find individuals listed as born in Africa.

THis feature is also useful when you are looking for TENNESSEE people "lost" in xxx county Kansas -- you can search the county, names blank, and get all the people born in Tennessee or wherever -- helpfuul sometimes to pick up mis-spelled or mis-indexed families.