Thank you for that interesting information. This coin came this way - I call it an "eBay Patina" I've had it for a couple of years in a flip and it appears to be very stable, whatever caused it.
Many of the older ones here will not have caught that song reference in the title. I, however, now have it fully embedded as an earworm- merely from reading the text- so here it is for the rest of you.
Blue patina is beautiful but can be rare. This is one of my favorites, a vibrant turquoise Philip II AE Several years back, I got this in a lot of ancient Chinese cash - regrettably I sold it, but at least have a picture. This is the in-hand appearance in natural light. Can't remember what type of mineral deposit this is