So, why did I call marketing terms “useless”? Really, a marketing term “urban fantasy” or “cozy mystery” pretty much says it all, narrows the audience enough for it to be manageable and, most of all, sounds very smart. God knows I’ve been wanting to write steampunk just because I adore the sound of that word. (And I haven’t given up this idea, just as I haven’t given up an idea to write a heavy doorstopper book.)
These marketing terms are necessary for publishers and bookstores. They give next to nothing for you as somebody who’s trying to build a personal brand (and convince people to buy your books) from scratch, using a blog and perhaps a Twitter account as your tools.
Why?
Well, because people pretty much know what they like to read, in these terms, already. Sure, at some point you’ll mention your genre, but it cannot be the basis of your personal brand. You’ll have to be more specific. And no, I don’t mean that you should say you’re writing “steampunk mystery with a dash of coming-to-age romance” instead of “steampunk”. I mean that you need to be more specific about things people will really get from your books.
So you’ll need to figure out what are the recurring qualities of your work. For example, “fast-paced”, or “dreamy”, or “cinematic”, or “funny”, or “crazy”, or “sophisticated”. That’s not easy since you almost never write what you intended to write. Having written a teenage outburst instead of philosophical treatise… Well, it hurts, and it takes time and courage to recognize. If you have a trusted reader, this is where you ask for help, too, especially since your trusted reader can point to things that you thought were too obvious to mention.

The funny thing is that you don’t need to mention this in your blog at all. This is your hidden knowledge, your secret recipe. And you keep it secret because people who’ll read and like your books won’t always read and like them for their strengths. All you do is act like a person who wrote your stories, and truthfully so.
That’s harder, and also way easier, than it sounds.