It appears that the phrase "take the black" meant something in Fantasy before GRRM's admittedly brilliant use of it regarding the Night's Watch. Back in 1976, to "take the Black" at the College of Caithnard means one has achieved the highest rank of riddle-mastery, for there Riddle Masters wear black robes. (How I had forgotten this fact I do not know.)
What is the first and last rule of riddle-mastery? Answer the unanswered riddle.