A blog is an online journal.
A post is an entry in an online journal.
If I write an entry, I’ve created a post, not a blog.
To relate it in SAT terms:
A blog is to a book,
What a post is to a page or chapter in that book.
If I write a single entry (or even two or three entries) about a topic, then I have posted about that topic, and when you reference that topic you can say, “Here is a blogger who has a post about XYZ.” It would not be accurate to say “Here is a blogger who has a blog about XYZ.” Just as it would not be accurate to pull the “C” encyclopedia off the shelf hand it to someone and say “Here is a book ... it's about Creamsicles.”
I say this because I care about you. And because it is good to be accurate and up to date when we reference current issues or trends. And hey, it took me some time to figure it out myself. You’re welcome.
If you’re not my mom, you can ignore this post.