There was never a 'regular black crewmember' that had lines of dialogue. However, by the third year of VOYAGE you could see a black or latin crewmember in crowd scenes or whenever the sub was going through 'lock-down' where they'd be shots of armed crewman walking the halls searching for the monster, mad scientist, or saboteur of the week. Not a good representation of a future intergrated Navy, but again the Seaview was NOT Navy per se, just followed some Navy protocols. It was primarily a research submarine with political ties to the US Navy.

At the time the series aired there were very few African Americans in the submarine service. Even today the number of African American in the submarine service is still quite low. In fact it is far below the number (percentage wise) of those in the US Navy as a whole.

Ever notice how there were no blacks in the crew..but set at a future time - RBONZOBOY5
I didn't, but now that you mention it, I do.
I suppose we shouldn't be surprised.
Gene Roddenberry
had to fight the network tooth and nail to get a racially diverse crew for
Star Trek (1966)
. Obviously
Irwin Allen
had other priorities when casting the crew of the
Seaview
.
Coincidentally, the two creators died just over a week apart in the autumn of 1991.