From the official website of the Cabildo
http://lsm.crt.state.la.us/cabildo/cab11.htm

Oscar J. Dunn was the first black lieutenant governor of Louisiana, elected in 1868 and serving until his death in 1871. Dunn was born in New Orleans, learned the plasterer's trade, and rose from private to captain in the Union's First Louisiana Regiment of black troops.

If you contact the Cabildo perhaps they will provide you their sources. Hope this helps.