Would summarizing peer-reviewed articles on a blog be frowned upon?
How deep are your notes?

I can't give legal advice, of course, and application of "fair use" varies from country to country. In the United States, I can't imagine you ever getting into legal trouble for summarizing your notes at the level of roughly an abstract (i.e. one paragraph), perhaps a few summary paragraphs. Keep in mind that the actual abstracts are virtually always freely available. So the value that the publisher must protect is the detail in the actual paper, not the high level findings, etc. 

If you're providing your own point-by-point commentary then my understanding is, that increases your flexibility under US fair use doctrine. Vigorous discussion and debate is part of why we publish, and citing someone verbatim (within reason) to reply to their points is generally smiled upon. So I'm pretty sure there's no legal problem with directly quoting a few sentences to respond to a point, as long as the total of all citations isn't more than say 10% of the paper.

Moving from legality to ethics, I see no problem unless you're quoting so much that most people won't bother to read the original. You're attributing sources so what you're doing is honest.

Practically, in US academia blogs are usually pretty worthless for hiring, tenure, and promotion decisions. I would consider them either a healthy hobby or an unhealthy time sink.