I tried to do a little investigation, but I came up with nothing. I could find no link between user ages (I found both adults and teens with both versions of the symbol), nor could I find any link between whether or not they're online (some people with the blue version were online, others hadn't been online for hours, and same for grey version). I noticed that it actually isn't static, as I saw someone's symbol thing go from the blue version to the grey version after I refreshed a page. It seems to have nothing to do with when their latest blog post was either. I used my browser's inspect element function and found that the blue version is named "blog_on@2x" and the grey version is named "blog_off@2x" but that doesn't exactly give any answers.
Now curiosity is also killing me, because none of the things that make sense seem to be the answer. XD