SAN DIEGO — The San Diego-based Navy command in charge of the service’s bases across California and Nevada is changing hands, as one Navy admiral heads into retirement and another takes the reins in a role sometimes described as the “Navy Mayor” of San Diego. Rear Adm. Stephen Barnett will assume command of Navy Region Southwest at a ceremony Friday at Naval Air Station North Island, the Navy said in a statement. He most recently served as the commander of Navy Region Northwest in Kitsap, Wash. and was the commanding officer of Naval Base Coronado from 2015 to 2016. Barnett is the third Black officer to serve as San Diego’s unofficial “Navy Mayor” and the first to do so as commander of Navy Region Southwest, which was established in 1999. Rear Adm. Benjamin Hacker held the title as the commander of Naval Base San Diego in the late 1980s. Rear Adm. Samuel Gravely, Jr. — the first Black man in the Navy to achieve the rank of flag officer — held the title from 1975 to 1976, when the command was known as the Eleventh Naval District.