How to fix when Mac fails to reboot, instead hangs at a black screen, while unplugging USB devices continues boot?
After a few hours of fiddling with USB and USB devices I found the solution.

In "Apple menu" > "System Preferences" > "Startup Disk" there was no longer a disk selected that I would like to use to start up my computer from.

After selecting the corresponding Mac OS X system, I could reliably reboot again, without pausing/hanging at the black screen during boot.