If you have absolutely no idea what the theme of your blog should be one suggestion is to look at several indexes of large portals on the Internet and see how they categorize themes and topics. Clickbank at www.clickbank.com provides a good example of what the main topics on earth are. However in general popular blog categories include news, society, culture, politics, religion, ecology, commentary, technology, business to business, art, information, recipes, web site building, humor, how-to, troubleshooting, rants and product reviews.
There are of course some topics that are more difficult to write about and that really do require some credentials or authority. For instance many people are tempted to write about politics, however the problem with writing this type of blog is that you can seriously turn off prospective buyers by maintaining certain opinions. The same goes for religious, medical or technology blogs. The most successful blogs are those who are written by someone who has some kind of expertise about a subject, otherwise you are just another weirdo on the Internet with an opinion.
The idea is to provide expert content on your blog; content that is backed up with research and facts. Remember that you are competing with the real news when you write a blog so it should be as well researched and credit given where credit is do. This means no plagiarizing or paraphrasing other people’s ideas. One mistake that beginning bloggers make is pitching ideas from other sources that are well recognized by readers. Unique information that is borrowed for the purposes of making yourself look brilliant usually makes you look very unethical. In fact the only thing it identifies you as is an expert plagiarist. As a rule of thumb, if you are printing and idea that has been printed before, it is not unique!