Justice League of America #110--A Christmas-themed issue, lol
[In the words of Luke Cage, "Sweet--"]
On-sale Date: 12-06-1973
Cover Dated: March 1974.
Mal Duncan debuted in 1970, but didn't become the Guardian until 1976. Tyroc appeared in 1976. Black Lightning wasn't created until 1977.
John was an active Green Lantern in December 1971/January 1972 and worked alongside the League in 1973.
You can debate whether or not Mal or John was the first black superhero at DC, but since Mal didn't technically become Guardian until 1976-- I think John has him beat as DC's first black superhero. (Black Racer debuted earlier in 1971 as a villain).
Black Panther debuted over at Marvel in 1966, with Black (Gabe Jones, 1963) and Asian (Jim Morita, 1967) Howling Commandos in desegregated military units being published earlier. Bill Foster appeared in 1966 but didn't become Goliath until 1975. Falcon appeared in 1969. Sunfire appeared in 1970. Luke Cage debuted in 1972. Storm appeared in 1975. Stan, Jack, and Marvel were ahead of the curve.