Suppose you were a single-blind paper reviewer (i.e. you could see the author's name, but they couldn't see your name) and you didn't obviously de-anonymize yourself during the review. If the paper gets accepted and published, and you are writing a review/summary of it (in its published form, not in any pre-publication form you have seen) on your blog after it appears in print.
Can you mention the fact that you were a reviewer for this paper?
Can you make comments about pre-publication versions of the paper? For instance: 'when submitted the paper lacked X, but other reviewers and I suggested it to be added and after revisions the author provided a full proof of X'.
I'd just pretend that you stumbled across a paper, rather than say you reviewed it on your blog.
And you should not make comments about the pre-published version of the paper. This could be embarrassing for the author, and seriously violates the trust embodied in the reviewing process.