You'll probably find this happening more often, because a good review site is a way for authors to get more attention for their books. It's probably worth your while to put a note on the blog saying that you don't accept books for review.

I understand what you mean about not liking to read a book specifically to review it. I've been posting occasional reviews in my blog of books I've read, and a couple of people have suggested that I join one of the review sites so I can get free books -- but I don't want to be under an obligation to write a review. When I'm reviewing books that I'd have read anyway, it's different.