The term "black helicopters" is occasionally used on discussion websites (especially Slashdot) after a conspiracy theory or other "risky" writing. It generally refers to a squad from a generic United States secret federal agency responsible for the handling of dissidents or people who know too much. For instance:
"The government seems to wish to regulate the entire Internet. Perhaps they are doing this to ensure that people in other countries hear only good things about the US, and bad things about the US' enemies! And the black helicopters fly toward my house."
There is an agency, the Federal Emergency Management Agency which has quite a few helicopters painted black, although there isn't really an agency that flies black helicopters to people's houses and spirits them away in the middle of the night.
However, if there is, I don't want to know. There probably isn't and it isn't worth investigating. I don't want to have black helicopters flying to my house to spirit me away in the middle of the night. God Bless America!