Is it impossible for new blogs to break into the big leagues? Have the Gawkers and TechCrunches of the world got blogging all sewn up? What blogs, if any, in the last year have joined the big leagues or made a dent in the domination of the established players?
Edited: Removed fluff, made main questions clearer
Trying to compete in an already-competitive niche is going to be, as you point out, near-impossible if you don't have "a fair bit of luck and friends in the right places."
However: there are plenty of niches out there waiting to be cultivated.
If I had your dream, I'd start from the other side. I'd be asking myself what knowledge do I already have, and am interested enough in to spend significant effort deepening, that would be of interest to some small-but-not-absurdly-tiny group of others?