I'm assuming you mean Prunus nigra, not niger - the fruits are edible, but do contain traces of prussic acid, in particular in the stones of the fruit. Children should not consume too many, nor swallow the stones. Can be cooked and eaten, or eaten raw, but not particularly palatable in raw state.

I agree with Bamboo, the best way to use them is to take out the stones and make jam with them.

The Italians do use them to make preserves with. Several in hotel I stopped at once outside of Florence.

In eastern Europe (Ukraine), the plain green leaved form has several named forms grown for their fruit. We sell a couple with re-branded anglicised names such as 'Golden Sphere' and 'Gypsy'. Heavy croppers too. Like many fruit, they taste best if left on the tree as long as possible. The P. cerasifera 'Nigra' that I tried in Italy were actually quite sweet raw though their summers are slightly better perhaps.