Based on my experience with blues, "double click" with blues is both of those. The click from the switch, and then bottoming out a few 10ths of a second later.

There's no need to bottom out with mechanical switches, but most people do out of habit with cheaper rubber dome switches.

I believe it's the sound of a tactile switch, and then the sound of the key bottoming out (which is not how you should be typing). It could also be the sound of the keycap hitting the board, but to me the first two are more likely.