Set colour to black on black for UTF-8 detection

In the auto detection string, the UTF-8 sequence and all the underscores look like garbage on some terminals. if \x1b[30;40m was sent after the first save cursor position, and the _ at the end of \x1b[0m_ was moved to the start of that (ie: _\x1b[9m), that should make the detection stuff look nicer.

I'm not sure what all the extra underscores are there for though aside from the one in the bottom-right corner.