I have seen blue Coralline algae with my own two eyes!
I started reefing over 25 years ago and was on the board of directors for a large reef society and I have seen hundreds if not a thousand tanks at homes and pet stores and I had never seen it or even heard about it so I was just blown away! It was in a tall established 150 Gal tank, right on the bottom about 6 inches from the front and center of the tank and I could't believe it! It was absolutely blue! Like a medium to darker blue with white sort of specks on it on that particular rock. I had bought a tank from her and she was going to give me some of it but that was a few years ago and I still have not been able to set up my tanks yet because of crushing nerve and structural damage thanks to a hit and run drunk driver.
I am a Coraline algae freak by the way and hope to someday collect as many species and colors (along with fish and soft corals) as I can, both encrusting and plating.
I hope all of you are healthy and safe and God Bless!