From Dana Milbank's column in today's Washington Post:
"The Christian Science Monitor had assembled the éminences grises of the Washington press corps -- among them David Broder of the Post, Maureen Dowd of The New York Times and columnist Mark Shields -- for what turned out to be a fascinating tour of an alternate universe."
An éminence grise (French for "grey eminence") is a powerful advisor or decision-maker who operates secretly or unofficially.
This phrase originally referred to François Leclerc du Tremblay, the right-hand man of Cardinal Richelieu. Leclerc was a Capuchin friar who wore grey robes. The phrase "His Eminence" is used to describe a Cardinal in the Roman Catholic Church, although Leclerc du Tremblay never achieved that rank.
Aldous Huxley wrote an English biography of Leclerc entitled Grey Eminence, and there is also an 1873 painting by Jean-Léon Gérôme, "L'Éminence Grise," which depicts him descending the grand staircase of the Palais Cardinal.