[Raellus]

There's another reference in canon that implies Soviet dominance of the Black Sea by

"The best Soviet troops had been shipped further south to Bulgaria and by May had managed to halt the Turkish drive." (emphasis added)

-v1 ref's manual, p. 25

Technically, one can "ship" things by land or by air, but I take this word to signify its more traditional meaning of "move by sea". Since the Romanian army hadn't collapsed yet- that wouldn't happen until September- the most logical way for those Soviet troops to have gotten to Bulgaria is by sea. This would strongly suggest that the Turkish and Romanian naval presence in the Black Sea had been eliminated by May, 1997.