In descending order of importance, here are the people who need to know that your blog exists:
People who Care About Your Topic
Blog Reporters in Your Field
Major Blog Aggregators
Social Media Aggregators
This seems horribly counterintuitive, because most people will tell you to keyword up so people who are looking for information on your topic will find you through search. But how do you actually rise in the search engines? By getting links to your blog from other, reputable sources.
The people who need to know about your blog most are the people who are *already* interested in your topic. They may be hanging out in forums, on mailing lists, Facebook or Google Groups, Twitter or IRC. Unless you find them and tell them about your blog, and about what it can do for them, they may never find it. Why? Because they are *already interested in your topic.* They have established hangouts and sources of information - they aren't searching for sites or information on the topic. These are the people who would comprise your core audience - if they knew about your blog.
Every industry, hobby and interest has someone, somewhere, collecting information and sharing it. Whether that person calls themselves a reporter or a linkblogger, online journalism has greatly expanded the sources a person can potentially use to learn about a topic. A few minutes with a tool like Alexa, or a Search Engine can lead you to the top sources for information and news in your field. Contact the editor - offer to write a piece on your topic, add some material to their encyclopedia, provide news for their feed or links through Twitter - do whatever you can to get noticed by them. This gets you on their radar and turns your blog into a source of information. News sites and news blogs often have a lot of followers - many only tangentially interested in your angle, but your name will get out there in front of a lot of people who would otherwise never have heard of you - and who *might* be interested if they knew your blog existed.
While you're out there looking for Links In to bolster your search ranking, don't forget blog aggregators in your topic area. Some are automatic, and will scrape your content through your RSS feeds (probably without permission) but others will require registration and/or submission of your blog. These aggregators are surprisingly excellent ways to get attention for your recent posts.
Social Media Aggregators like Reddit or Digg are incrementally useful if your topic is specialized, or you're entering a field with established opinion leaders. For instance, a Mashable post is going to get way more Diggs than anything I write here. It's not an expression of quality, but of quantity. If you have a small, but devoted following, asking them to share your links won't hurt. Don't expect them to bring in a gazillion new followers, but they might bring you incremental growth in people who actually care about your topic.
If you've done all the above - and you've got a blog built on a foundation of relevant content - the final step is a no-brainer. Search Engines will find you, because your keywords and phrases are well-placed, you have links in from reputable pages and people are looking for your blog, specifically. Set the other pieces up and the Search Engine piece will fall into place as naturally as it can. Be mindful of the relative size of your audience, the other sites and blogs with which you compete, so you're not delusional about the importance or impact of your blog on a Search Engine - and don't assume that being on Page 3 of a Search Engine is the end of your blog.
Work outwards from a small core of people who care, to build a solid blog audience that will support you in your expansion. Drop stones carefully into your pond and watch the ripples move outward to create a greater impact.