No treatment that works is available to amateur growers for Pear Leaf Blister Mite - control is usually to pick off worst affected leaves, but infestation by the gall mite which causes this does not usually affect the quality of the fruits. Rake up all fallen leaves and dispose of at the end of the season.
Pear Scab, though, is often confused with Pear Leaf Blister Mite - this will produce blackish brown scabby patches on the fruits and similar patches on the leaves. This needs treatment.

On one of my pears which is smallish with a thinned crop of huge pears. I have just scissored out the galls and left the rest of the leaf to serve the tree, as it's so small and needs all the leaf it can keep .Seems to have worked OK. Stamped on the inhabitants of the gall. Most of the midges in these galls have gone about their business by now.