Sandra Kitt's Adam and Eva was the first Harlequin romance written by an African American author about a African American couple, in 1984.  It's not the first black category romance - as far as we know that's Rosalind Welles' Entwined Destinies, which was published by Dell Candlelight in 1980.  There's also a rumor that Anne Weale's Blue Days at Sea is a "stealth black romance" - evidently the protagonists' race just isn't mentioned but the people on the cover are even whiter than usual.  (And they are usually really white.)