Wiktionary has it:
- (transitive, Internet) To blog something previously posted on another blog.
And the Urban Dictionary has another definition, which is mostly downvoted (2 up, 5 down):
a blog whose sole purpose is to post about other blogs
Basically, even if most people wouldn't recognize it, it's technically a word as long as someone uses it to communicate an idea. By definition, as well as grammatically, it is a word. Some people add to the definition to say that it must be widely used in order to be really recognized as a word, but in that case, the issue boils down to recognition and usage, not the word's status as a "word". As to recognition, I think this one isn't too hard to figure out from context, even when one has never heard it before; as to usage, it is uncommon, but all words start out that way. Blog itself was such a word once.