Hey Caroline -
It's sooo key for YOU to know who your target market is, because none of us can or should try to identify that for you. However, we can help you find the answer for yourself.

Some ways that you can identify your target audience is to determine who YOU like to work with. What type of industry, people, personalities, values, gender, etc. Determining this is just a place to start. As you begin to work with people, you will refine this.
As far as blogging - the purpose of blogging should always be to showcase your experience & expertise. It must be a strategic part of your marketing strategy. But it must also add value to the reader. However, remember you are marketing YOU as well. So, while I totally agree with Luke...occasionally throwing in some little snippet about you can be a good thing. For instance, you could do something like. "As a copywriter, it just seems to be that every typo, every grammar error, on a website jumps write off the screen at me. Last weekend, while I was up skiing in Vail, I was looking at the hotel's electronic marquis and I couldn't believe what I saw!" Then proceed to tell them what it was, and then offer them tips and educate your audience how they need to avoid the same mistakes.
Just a couple ideas to hopefully help get you started.