If you look around, you'll find that there is lots of advice out there in regards to how you should choose a topic to blog about. Some folks out there would tell you to focus exclusively on profitability. Go after the niches and markets that people are buying with the most intensity they will say. Seek out micro niches within these popular main niches.
On the other hand, you have those who tell you that it does not matter about the profits. What matters is what you are passionate about. These folks tell you to go after the things that you personally enjoy and have the most personal knowledge about. Therefore, it will be much easier for you to create content and products.
In the end, the wise person would try to pick the best of both worlds. While it is true that if you blog about a profitable niche in which you have no interest, it will be quite hard to generate useful content day after day as is required by a successful blog. Conversely, if you pick something you're passionate about but there is no market, then all the great content in the world will not give you a profit. So, you have to find a balance somewhere.
You have to realize that we human beings are different in terms of what motivates us as individuals. A lot of us get passionate about the process of building a blog and earning profits, in which case it would be easier to blog about something that you have no interest in as long as the results are successful. Other people have to enjoy the creation itself. In any event, you have to find something that you enjoy and are motivated to blog about daily.
The simple approach to this dilemma is to make a list of things that you're passionate about and of which you have the most knowledge. Next, make another list of the things relating to a blog that you are most skilled at doing. Finally, make a third list of the topics that you perceive as being the most profitable. Now take these three lists and find some ways to overlap them. The most ideal spot for you are those where all three lists overlap.
While this may seem like a lot of work for such a trivial approach, you will be surprised at how effective it can be. It is said that the best way to quiet your mind is to put what's in your head down on a piece of paper. This exercise is just the right remedy for doing this.