Hello! 

It's possible that the fruit are not developing because of poor pollination.  I don't know how the weather was while the tree was in bloom, but if below 65 and/or rainy then bees are not very active.  Most varieties of European pears are not self-fertile, so cross-pollination is required for fruit to set.  It is common for an un-pollinated fruit to begin developing but then drop early in the season.  Fruit drop can occur for a variety of other reasons too, like extreme temperature swings during bloom, but I'd say poor pollination is the most common cause in MI. 

There were also some freezing events this spring across the state, so i wonder if the flowers were damaged by a frost during bloom and therefore fruit are not developing or if the frost happened later and the fruitlets were damaged and are now turning black.  The fact that they turned black makes me believe this may be what happened as opposed to just poor pollination.  

There's unfortunately not much that can be done to avoid either of these problems, we're at the mercy of the weather!  If the tree looks otherwise healthy, sounds like you're doing what you can.  

The different varieties can cross pollinate, yes, but there needs to be good pollinator insect activity when they are blooming for the flowers to be cross-pollinated successfully.   

As far as the different fruit trees, cherries bloom before apples and pears, but different types of fruit trees, and even different varieties of the same type of fruit, vary in cold hardiness widely.  So having cherries doesn't completely rule out it not being caused by weather.

If there's a way you can send pictures, that would be helpful in determining if it could be weather or something else.

Sorry, we are not able to receive pictures through Facebook.