If you can focus on a topic that you already know something about, or at least that you're passionately interested in, you'll be far ahead of everyone else. Knowing a fair amount about a topic helps in multiple ways. For one thing, you'll have to do much less research. For another, familiarity just shows. You can tell by the way someone writes if they're intimately familiar with what they're writing about. Their words have that ring of truth and experience that even the best researcher can't fake.

Not only that, but a blog is not a short term deal. It's a longer term deal, if you will. If you're going to be doing this right, you're going to be living with your blog day in and day out for a year or so. Unless you are really interested in your topic, you could easily get sick of your blog a month or two into the game. That would cause you to quit writing content, which will result in your blog losing readers rapidly.