A blogpost is one piece of writing put up on a blog — one update, one article. People also write it as two words, blog post, and that spelling is very common; the one-word form means the same thing. As a verb, to blogpost is to put that writing online, though in real talk people more often just say post or blog: “Did you see her latest post?” The word is informal to neutral. You will meet it in tech and media, not in a very formal paper. Though actually, once the writing is up, “post” by itself usually does all the work.