In order to build a faithful following on your blog, it is important that people know what your blog is "all about." If your blog doesn't have a consistent theme that is useful to specific group of people, then there is little chance that you will secure a group of faithful visitors to your site.
On the other hand, you should seek a balance between keeping your message limited, but not too limited. You want to corner your niche, but you also don't want to paint yourself into a corner. If, for example, you are marketing yourself to a particular region of the country, then you might think about how you might eventually expand your interest base to the rest of the country, or even the world. It is also possible that your blog will become so popular that you will want to expand your reach into other areas of interest or industries and thus expand your money making opportunities.
This may or may not be possible for you in light of your current marketing strategy, but it is worth considering when thinking about your long term money making efforts online. If you do not have the option of expanding your current blog's reach, then you could always create additional blogs. The downside to starting completely new blogs is that you will have to launch your marketing efforts almost from scratch -- I say "almost" because you can springboard from your current blog's success by advertising your new site on your old site.
In any case, if you blog about your interest and stay consistent then you can't really go wrong. Other people with similar interests will be drawn to your writing and eventually, you will gain a large following regardless.
Along the way, if you should run into a creative brick wall, a.k.a. "writer's block," then you can always use Spinbot to spice up your current content, or possibly old content that is gathering dust online or on your computer. Thinking of new ways to talk about a strategically limited subject can be difficult, but that's why Spinbot was created. Spinbot will turn your thoughtful content into additional, human readable text.
Go ahead, give it a try.