By the way: What happens if the user uses a system with a refreshrate of 80 Hz. Will Blitz change this system to 60Hz then or will it run at 80Hz and use a delta of 60/80?
Blitz takes the hertz number you specify in the Graphics command (if no hertz parameter is specified, it defaults to 60). It tries to open a screenmode with that number for the refresh rate, but if it can't it will open the first mode that is returned from the video card driver's list (usually 60 - hopefully that will be changed to the closest match in the future, rather than just the first returned value). In any case, Blitz continues to run your code internally at the hertz you specified. There is a way to turn this off, it's an OpenGL command which turns off the framewait in Flip. I don't know it off the top of my head.
If you specify 0 for hertz, Blitz uses the default/users desktop refresh rate.