In Russia, there are four varieties of Leningrad sweet cherry: Leningrad Yellow, Leningrad Pink, Leningrad Red, and Leningrad Black. Fedco sells Leningrad Black (it’s the most popular one of the four). It’s self-sterile, so you will need another sweet cherry for cross-pollination.
Sweet and tart cherries have very different flavor profiles. In my opinion, one is not a substitute for the other. I like them both, but it’s like apples and pears — each one has its unique flavor.