Whatever the subject you wish to center your blog content on, there must have been other bloggers in that field.
If your blog content contains well targeted SEO optimized keywords, your blog will rank well on search engines. That means that your blog will have to have high visibility amongst the thousands of similar niche blogs. Some bloggers create valuable content, yet they do not have adequate visitors nor make enough money from it. This is because they either do not make research on keywords or do not use them appropriately.