How much do I reduce the sugar when using a sweeter fruit in a pie filling?
It's mostly a matter of taste. In theory you could leave the sugar out completely, it's not needed to "bulk up" the filling, but that would probably not match our expectation of "pie filling". Your own question mentiones blueberries, which have about the same degree of sweetness as Bing cherries. I would start with the amount from your blueberry pie recipe.

Apart from that, consider adding a tart/acidic flavour aspect because it nicely complements the sweetness and intensifies flavour. (Probably one of the reasons many recipes use tart cherries.) A dash of lemon juice or, if you have it, some fresh red currants (also in season right now).