The first GOP black male elected to the US Senate was Edward Brooke.
Unbelievably, he was elected in Massachusetts in 1972 and served one term before being defeated by Paul Tsongas in 1978.
There may have been a GOP black male who was elected to a Senate seat during Reconstruction...but I don't think there was.
Mr. Cain would be the 6th African-American Senator elected. The first was Mississippian Hiram Rhoades Revels in 1870, serving for 1 year. The second was Louisiana Governor Pinckney Benton Stewart Pinchback in 1873, and was also elected simultaneously to the House, but he was denied his rightful seat in both. The third was Mississippian Blanche Kelso Bruce in 1874, serving a full 6-year term. The fourth was Massachusetts Attorney-General Edward Brooke in 1966, and he served two terms (the longest serving African-American Senator to date), before being defeated for a 3rd term. The fifth was Illinois's Carol Moseley-Braun, the first and only woman and first and only Black Democrat elected, in 1992.