If I was interested in assessing someone's caliber, I'd rather look at their Github than their blog. Anyone can write a blog post, not everyone can write good code.
Blog if you have some insight into technology that hasn't been already elucidated or if you disagree with what's out there.
Having said that, I plan to start my own blog at some point. What will I be writing about? I'll start with reviews of technical books. Why? Because I have read a couple since the beginning of the year that I was unhappy with and had to return, but bought them in the first place because other people said good things about them. Hopefully if someone in the future considers them they will come across my view and take it into consideration.