You are probably aware, if you've grown Swedish ivy (Plectranthus verticillatus) at all, that the sap turns orange when exposed to air. It'll even stain skin orange, in fact, which is one of the more annoying things about the plant.
Plectranthus oertendahlii, on the other hand, doesn't do that. As far as I can tell, its sap starts out clear and then stays clear. But it does something similar. When the flowers get wet, like for example when you're watering them in the bathtub, they'll turn the water blue.
I realize this photo doesn't show the blueness coming from the flowers. You're just going to have to trust me that that's how it happened. I first noticed this a few weeks ago, and wondered about it, but so many things get sprayed around the shower when I'm watering that I thought possibly it was unrelated to the plant. But, lo and behold, there the blue water was again the next time I watered the plant.
I doubt this has any practical applications. I don't know whether it will dye fabric (I'd bet not); I know it doesn't dye skin. I have no idea what part of the flower is responsible. I don't know how dark the color can get. It's pretty pointless to know this, or to tell others about it. But there you go anyway.