A union is basically an attempts to form a monopoly over labor. As with any monopoly, it needs two things to work: 1) A "moat," or a non-reproducible qualitative advantage, and 2) high barriers to entry. Blogging has neither, which is its appeal. There's little that bloggers - even the best - do that can't be done by a million other people with time and interest. And the blogging universe exists precisely because there are virtually no barriers to entry.
What on earth a blogging picket line would look like, anyway? Denial-of-service attacks?
Then there's the fact that such a monopoly would produce what all monopolies produce - a lower-quality, lower-quantity, less-responsive, less-innovative product than we had before. Kind of like the MSM>
Which is also why we have blogging.
There must be a reason why there aren't any well-read economics or business bloggers behind this idea of a "Bloggers' Union." Aside from the obvious.