Here are a few tips to consider before choosing a niche topic to blog about:
1. Choose a topic that you are genuinely interested in. Don’t choose a topic because someone else may be making money on it. Consider the time its going to take to build up a following. Are you passionate about the topic you chose? Will you be just as passionate a year from now? If the answer is no or even I don’t know then choose another niche topic.
2. Is the niche topic popular? Remember that the blogosphere is a global community. Are there enough people searching for your niche topic to make it worth your while?
3. Is your niche topic part of a fad? Find something that people will be searching for a year from now and not just some flavor of the week.
4. How much competition does your niche topic have? Competition is a good thing because it means the niche topic is popular, but being too popular means you will have a lot of work cut out to get your blog circulating. It’s not impossible.
5. know your competitions and deliver what they are not. Opposing view points are just as popular.
6. Do you have enough content to deliver on a regular basis? A sure way to lose viewers is to not deliver on the promises made by the blog.

I’ve written more about how to find a niche in my hubpages article.