Not strictly a gardening question

It is a genetic mutation and whilst not rare it is uncommon. It is an incomplete form of albinism but not a fully blown one as that would be white feathers and red/pink eyes.

I love birds, well most of them, and I knew it wasn’t usual. Part of me was convinced that it was a dead relative visiting trying to tell me something! Another part couldn’t see why it would be a blackbird anyway. The poor thing had flown into the window and died in my hands.
Another thank you.

It could also be an indication of age in some birds. Example, while on a fishing trip I encountered a ornithologist who had a red tailed hawk who was ready to be released now that it had recovered from an injury. I noticed that the hawk had blue eyes. He explained that the bird was a juvenile and that their blue eyes turned off a territorial aggression response in adult hawks to allow the juvenile to hunt in their established food range. The eye color would change upon reaching maturity allowing the adult to attack any red tailed trespassers. Of course this may not be the reason in your case but I thought that this encounter I had with the ornithologist an interesting example.

Loosestrife, that was an interesting suggestion and sounded quite plausible to me.
Thank you.

certainly some of my nephews had blue eyes at birth and it was up to a year before the final brown eye colour stabilised. The midwife told me that white babies were usually born with blue eyes. Both mine were and one stayed blue and the other had green eyes by 9 month.