I added maps but I guess you had hit 'quote' before they went up.
As I said, Russia has ports all over the Black Sea. I understand you got your info from another poster, but they are ignorant of the geography of the Black Sea. Mariupol is in the Sea of Azov, which has one tiny outlet into the Black Sea. That outlet is completely controlled on both sides by Russian Territory. If Russia wanted, they could choke off the entire sea of Azov. So Mariupol is not a factor in that regard.
The Black sea itself has one tiny outlet into the Mediterranean, which Turkey can and sometimes does cut off.
Turkey has closed off the Bosphorus and Dardanelles straits to warships from any country, whether or not they border the Black Sea, following Russia’s invasion of Ukraine. The strait closures will still allow warships through if they are returning to a home base in the Black Sea, according to...
news.usni.org