Er... depends on your definition of "channel".
If you mean "a way to transport cargoes from Baltic to Black Sea by water", then it exists since 1952 (if I'm not mistaken), when Volga–Don Canal was built at last (you go from St-Peterburg to Volga by Mariinsk Canal System (1810), down the Volga river and then to Don and Azov).
If you mean "a way to transport cargoes from Baltic to Black Sea by water in a more or less straight line" then it's more or less ASB.