Sampson W. Keeble was the first African-American to serve on the TN State Legislature. He represented Davidson County from 1873-1875. Earlier this month I was out at Greenwood Cemetery and happened upon his grave.
As you can see, he is buried with his daughter and son-in-law. The Tennessee State Library & Archives has an online exhibit of black legislators from Tennessee in which Keeble is included. His bio has several details about his personal life and legislative career.
The bio mentions that he last appears in the Nashville City Directory in 1886, and now that Ancestry has Nashville City Directories now from 1879-1899, I was able to quickly look him up. Looks like he lived at 100 N. Spruce St.
Given how new his headstone is, and given that a previous bio of him indicates it was now known where he died, I suspect this marker may have been recently done, perhaps in conjunction w/ the TSLA exhibit?