Turkey seems to control the only access to the Black Sea?
The Montreux Convention Regarding the Regime of the Straits of 1936 controls passage through "the Turkish Straits."
Article two provides "In time of peace, merchant vessels shall enjoy complete freedom of passage and navigation in the Straits, by day and by night, under any flag with any kind of cargo."
The passage of civil aircraft is also permitted along routes set by Turkey.
There are further provisions relating to the passage of warships, with those of Black Sea states (needless to say including Russia) given far greater leeway than those of non-Black Sea states
http://sam.baskent.edu.tr/belge/Montreux_ENG.pdf
So what does this mean, among other things: Russian aircraft can be crated up and shipped on merchant or Naval Auxiliary vessels through the straits to Syria (likewise on transport aircraft), likewise Russian warships can transit the straits (in either direction). All this goes on until either Turkey or Russia declares War on the other.
Of course this totally ignores Russian Aircraft transiting Iranian and Iraqi airspace to go around Turkey.