What should I blog about is a really common question when people first start blogging or when writer's block sets in. And to be honest there is no single answer here as every blog is different.
If you wanted to oversimplify the answer we could say you can blog about anything!
However, there is a catch to this, because if you write about anything you may fall into a trap of either attracting no visitors or worse yet losing your current visitors by blogging about something disengaging or irrelevant.