Source?
There's NO WAY Turkey could close the Balck Sea to Russia. But they could close the Bosporus Strait.
For a little while.
But that strait is covered under the Montreux Convention Regarding the Regime of the Turkish Straits of July 20, 1936.
It would be a clear act of war on Turkey's part and would offer legitimacy to any Russian military action to re-open them.
NATO WILL NOT SUPPORT THIS.
So consider this suspect source, to be sure:
TURKEY BLOCKADES RUSSIAN SHIPPING, BLACK SEA FLEET COMPLETELY CUT OFF