In my opinion blog posts have no scientific quality at all, since they are not peer reviewed. They may occasionally have quality, but that's another matter.

To be fair, some fringe journals have no scientific value either, because they do not peer review properly, but they are easy to spot and everybody in the scientific community knows them.

On a side note, originality is one of the key requirements for publication in a reputed journal and I have not ever seen a blog post that made any scientifically original claims. But maybe that's because I don't read blogs very often.