I've had Celia for 2 and a half years and like a year ago this blue blob just appeared randomly on her. She is lively as ever and seems to be unaffected. But I think I also noticed this on my other really old fish, Moe but it was less prominent on his white body.
This is a older picture of her before she had the mysterious blue blob:
This seems to be nothing urgent... but I want to make sure it's not going to be.
Also, she has lived with another female at one time [I moved them apart after a week because I didn't realize the sorority rules] and she has attempted to mate with the other fish who I noticed the similar spot on [they did not successfully mate] and now she lives in a community tank with about 30 guppies in a 10 gallon