What is this substance that turns blue when I spray bleach on it?

I’m cleaning my bathroom, spritzing bleach pretty much all over the place, but when I sprayed down the toilet, previously invisible streaks around the tank and seat turned a dark blue when I sprayed it with bleach. There’s a fair bit of grime on it, but the streaks weren’t even noticable before. Now that I’ve sprayed the whole thing down, the streaks have turned a really dark blue. After about 10 seconds or so they fade, but what substance turns a dark blue when it comes in contact with bleach? The bleach I’m using is Cloralen.