I am not sure, but a plastic bag may make it worse by keeping the heat in rather than keeping the sun out. I have seen shade cloth covering the whole plant used but never a bag over the fruit itself. Sun scald for me is generally whitish roundish splotches; yours look like bruising. As my plants grow larger with more foliage sunscald is less a problem.
I agree that blossom drop is most likely due to heat stress and it happens to us all. You do have nice fruit on the vines and will do OK from the looks of your photos.
