When students gets expelled, they are granted a black trash bag by the school as a parting gift to be used for cleaning out the student's locker.
That kid wrote a note about conducting various illegal and/or unethical exploits. The teacher found it, and an hour later he was black-bagged.
Government paid thugs throw a black bag over your head and kidnapp you whilst beating the piss out of you and making you disappear for ever. (as in V for Vendetta)
Tommy said some bad stuff about President Obama and he was Black-Bagged.