I just got a reply on another forum - apparently it's completely normal for guppies and nothing at all to worry about!....Phew!

Hi there Sorry I'm not much help but I have also noticed that all of my 4 female guppies eyes have all gone completely black. I'm really worried and I can't seem to find any information about it anywhere! They seem to be behaving normally maybe just a tiny bit slowed down. I think it could be stress ...