I've seen people, including people who write for a living, say they wrote "a blog" about this or that topic, when they obviously mean a specific post.
I find this incredibly grating. Dictionaries don't seem to include this sense, which leads me to think this is a misuse of the language.
On the other hand, blog is a portmanteau of web and log, and one could (I think) use log to refer to a specific entry in a record.