blue goo
n. Proposed nanotechnological machines that would monitor and control other machines to ensure that their replication does not get out of control.
The nano-enthusiasts also occupied themselves considering whether such "gray goo" might be effectively countered by "blue goo," policebots that would form a nanotechnological immune system.
—Bill McKibben, Enough, Times Books, April 01, 2003
Mr. Joy said mankind is on the threshold of creating tools ''that are so powerful that they fundamentally threaten the social contract,'' the ceding of individual liberties to the state in return for the benefits of civilization. ''We are likely to empower those people who have an agenda,'' he said, and he was not referring to a Filofax. But if the best solution is to put some kind of governmental or supragovernmental authority in charge of deciding what science is good and what is not, then we would do better to hope that the invisible hand of Adam Smith's marketplace provides a solution. Which it might: A possible solution to gray goo is blue goo: tiny self-replicating police robots that keep the other ones from misbehaving.
—Mitchell Martin, “Technology's Little-Heeded Prophet,” International Herald Tribune, October 23, 2000
1989 (earliest)
Gragu can be recognized by what it does (nanovandalism). It can hide all it likes, but eventually it has to attempt to perpetrate some crime. Otherwise, it isn't gragu, is it? Imagine the world INFESTED with repair and defense nanoagents, at high density, ready to spring to action at the first sign of inimical activity (NAT MAN and ROBIN? :-) :-)). Of course, the problem then becomes the reliability and security of the nanopolice. Can they be trusted? Subverted? Could a traitorous strain be introduced that would out-replicate the bugs in blue, displacing them from the target area, and then striking? (Oh no, have I invented a new term: "Blue Goo" (The Nanopolice))?

Blue Goo may work—if it can obtain and maintain a technological lead over the purveyors of Gray Goo.
—Alan Lovejoy, “Re: Miscellaneous,” sci.nanotech, May 31, 1989
When I posted global ecophagy back on April 24, I mentioned the gray goo problem. To recap, if nanotechnologists get their way, some time in the future (30 years? 50 years? no one knows), some or all manufacturing will take place at the nano (one billionth of a meter) scale. This will be done by assemblers — unfathomably tiny machines that can be programmed to build just about anything atom-by-atom; this includes copies of themselves made by replicating assemblers or nanoreplicators. If there was a Worst Case Scenario Handbook for nanotech, it would include the possibility that this replication would somehow get out of control and the resulting trillions of assemblers would destroy everything in sight, leaving only an undifferentiated mass of nanoreplicators: gray goo.

One "solution" to this hypothetical problem is adding extra nano-machines to the mix that would prevent the replication from getting out of hand. Since these machines would be effectively "policing" the other assemblers, one nano-wag (see the earliest citation) dubbed them blue goo (which certainly puts the phrase "thin blue line" in a new context).

Are there other nano-goos out there? Why yes there are, thanks for asking. There's golden goo, nanobots designed to mine gold from seawater; khaki goo, nanotechnology used for military purposes; and red goo, a collection of replicators designed to cause harm (a kind of nano-terrorism).