I don't see the blue box anywhere on that page, but it looks like the gray box you have is on line 271 of editor.css (with Firebug installed, right-click on the element you are curious about, and select "Inspect element with Firebug").
I don't write the CSS for the themes, and I'm not even that good with CSS; I write the code that makes it possible for the designs from the Joomla club to run on WordPress, and I do my best to add extra capabilities that Joomla doesn't have.
This tutorial should help you understand how to use Firebug; that's what I do to answer the CSS questions, so there's no reason for you to be forced to wait on an answer from me.