Question

Why might a blog be considered an unreliable source?

Answer 1

It may not exactly be a primary source because it's not exactly like, for example, research paper. Research paper has been reviewed by many peers and has been tested to see if it's all true, so you can put more reliability on it.
A blog is just put up on the Internet without peer review or anything. It's raw and nobody knows what's true and what's not.

Answer 2

A blog could be considered an unreliable source because the information written and shared isn't confirmed by a professional and isn't published as an article, which is typically reliable because it's reviewed and fact checked. Blogs aren't official and the information could have been taken from anywhere which makes it very hard to know if what you're reading is the truth or not