Bleen is an artificial
predicate, coined from "green" and "blue" by philosopher
Nelson Goodman[?] in one of the seminal works in the
philosophy of science,
Fact, Fiction, and Forecast. The predicate is used to illustrate what Goodman calls
the new problem of induction[?].
Specifically, "bleen" means "blue before January 1, 2000 and green on or after January 1, 2000". See also grue.