To me, a blog is something informative for readers so they learn from it, interesting enough that they will return, and entertaining for visitors because it is creative and different, and visually appealing.
To bring visitors back, you must keep your blog interesting and fresh, with new information posted frequently or at least periodically. I created a blog a couple years ago and it's all about eLearning, computers and technology because it's something I love to write about and I enjoy writing instructional articles to help others on technology topics.
I think in order to have a successful blog, you have to write about something to have a passion for, and something you love to read about yourself.
Good luck with blogging!