It may have been caused by someone using "diurex" "the gentle diuretic" (water pills) I knew several people years ago who used this stuff. (myself included ) The box back has a note at the end of warnings. "NOTE: The appearance of blue tinted urine which may occur after taking DIUREX Water Pills is a normal and temporary effect. (Yes, I still have an old box hanging around, though I can't remember the last time I used it.)

I never had mine turn color though. And it could be from the dye as the pills were blue.