I remember it a bit differently - I thought it was 'blue ice' from a leaking lav service pane
Better memory than me
td 
You're correct of course, Northwest Flight 5 report,
THE FORWARD LAVATORY EXTERNAL SEAL WAS MISSING THE SAFETY BUTTON AND THE INTERNAL SEAL WAS IMPROPERLY INSTALLED ALLOWING FOR LEAKAGE WHEN THE AIRPLANE WAS PRESSURIZED. THE FUSELAGE EXTERNAL SKIN AFT OF THE FORWARD LAVATORY SERVICE PANEL WAS STAINED WITH BLUE FLUID