For clarification, the object designated AT2021biy has not been classified as a supernova but rather a Luminous Red Nova or Luminous Blue variable:
The response to my communication with Andrea Pastorello, one of the authours, is as follows:
“This is surely NOT a supernova.
I’m not sure 100% it is a LRN or an LBV-outburst, we need to follow it to precisely classify it.
It is called *red* because it becomes red (cold) very rapidly, much more rapidly than most transient types, and will reach a mid-M stellar spectral type
after a few months”.
I hope this clarifies why the object appears blue, well shown in Peter Tickner’s excellent colour image. It would be nice to see an image when the object eventually turns red. However that may be very difficult as it could be quite faint.