Turkey and Russia are a great alliance, but the challenge comes in striking at the last moment. If you wait too long you're ally will be too large to attack or they will attack you first, but if you strike too early then you will run your alliance and risk both of you going out of the game early.
The two countries work well together since they can divvy up the Austria-Hungry-Neutral peninsula and Russia is even safe to let Turkey take a larger piece of the pie since Russia has the entire northern part of the board to work with. The situation would be a little different if Germany were occupied and Austria-Hungry were able to strike an Alliance and push back at Russia.
The Black Sea works as a perfect neutral zone providing at least a 1 season warning to either country of an attack. 1 key to the sea however is having two fleets on it so that one can support the other both in moving or bumping someone else out. I think that if 2 fleets aren't established early on by Russia that it could easily be viewed just as aggressive as actually moving to the Black Sea.
Alliances in Diplomacy are key to a strong start and are required at least for the first 2-3 years until you are strong enough to support yourself but non-aggression packs are key to victory so that you are not fighting a two front war. I also have learned from this game that a good alliance is between 2 strong players who both benefit equally from one another like this game with Turkey and Russia neither getting in the way and always taking turn with who supports whom. England and France in this game however had a poor alliance blocking each other from crucial points, not communicating, and England crippling France by reducing stars on more than one occasion. The only benefit the alliance saw from one another was non-aggression which allowed them to leave the exposed side to each other completely unoccupied.
In war always be ready for your ally to turn on you and support as frequently as possible when a strike could occur even if they do not it is better to be safe than sorry.