Pamela is right. Any topic will work. The key is that you have to immerse yourself in that industry, so you should be passionate about it. You can't just depend on passively posting on the blog—the blog is just your entry point.

You will need to build relationships (online and offline) with others in your topic and eco-system. You have to become known in your industry. Then people will follow your blog, you can speak, etc.

Once you become involved in the industry, you will have opportunities to make money.

But, if you want to do it right, and go big, ignore making penny ante money through google (or anyone's ads) or sponsored posts. Focus on building credibility and making real links with people in your industry.