When is a blog considered to be a commercial one, especially when it is under a Creative Commons license?

I previously licensed one of my blogs under a Creative Commons license (Attribution-Share Alike 3.0), and there are no advertisments on my blog, as I did not sign up for a service similar to AdSense (nor am I intended to do so in the future). Is this considered to be a commercial activity?

It does not sound like it is commercial unless you are using it to promote a specific commercial advertisement or you are making money from it in some other way. If it is not your intention to directly profit it is not.