This could be a fungus called Quince Blight, very common where the wind blows across a site distributing the spores.

Bordeaux Mixture (copper based) applied to the leaves is supposed to slow it down but spraying a large tree is impractical. And the rain washed it off so you have to repeat the application.

when you say they're turning black, do you mean the surface of the leaf is turning black, or do you mean they've got black marks within the leaf tissue? Photograph would be useful - could be Pear Scab. Any other symptoms such as curling leaves, or signs of black marks on the fruitlets?