I have had a pear tree for many years and have had this problem in the past. It is caused by a pest which lays its eggs early in the spring - the grubs then grow within the developing fruit. The solution is to spray in late autumn, to kill the over-wintering pests or, at the latest, when the buds very first appear, before the blossom breaks. You can get organic sprays from the GC and I have always found them to be very effective. Unfortunately, once the developing fruit goes black, you will have lost most of this year's crop. It is not a disease of the tree, though, and spraying in late autumn should prevent it happening next year.