According to the answer of https://stackoverflow.com/questions/15597936/blocking-time-in-google-chrome:
"Blocking" is the time the request spent waiting for an already established connection to become available (i.e. we're reusing a connection that may be used to transfer another resource at the time the request was started).
But why is it that when I refresh a page, the very first request is blocked?
refer to the screen shot: