Canada #919. Error? Any Help

Here's the image:
https://ibb.co/igXkev
Hey guys,
I'm a 16 year old collector and found this stamp in a random mixed lot. The stamp on the right is the same stamp, found in the same lot, but not half blue. What I am wondering is if the stamp on the left is some kind of printing error, as half of the stamp is blue. It's not a stain as it doesn't go into the white border, or into the black picture.
Any help would be appreciated.
Thanks,
R9191

Hi R9191, and welcome to the forum. What you found looks like a changeling, a stamp that has been in the sunlight too long or maybe chemically altered.

Peter

This particular issue is notorious for changing color when exposed to sunlight. Bileski offered the set in stunning color changelings caused by exposure to the Winnipeg summer sun.