Blueberry bushes losing leaves to pests?
Not sure why your leaves are disappearing. Most leaf feeding pest do not eat the entire leaf. Deer would eat whole stems and fruit. Leaf spotting is also nondescript. Most leaf spot diseases have bigger spots and usually distinct centers or margins. Small red spots on the leaves don't mean anything specific, more a general sign of stress. Lack of water would have burned leaves. Have you been short of water?
I notice you have a heavy bark mulch. I wonder if the problem is simple lack of nitrogen. Leaves are only light green I would expect darker leaves. Reddening of some of the leaves in the pictures look like lack of nitrogen or phosphorous. The mulch will use a lot of nitrogen and we recommend applying 2 or 3 times the normal fertilizer amount if you have a mulch.
It is too late in the year to be fertilizing but early next year you should fertilize. For homeowners I recommend a commercial rhododendron/azalea mix like Mir-Acid. Apply half during the early spring and half right after bloom.