Sudden knob color change - frostbite?

[GEESES Peanut Butter Cup]

The knob was black in the morning. After coming back from work, I saw the spot on its knob as in the photo. Did the goose suffer from frostbite? What should I do? (The vet won't be back for a while.)

 

[sourland]

It almost looks as if it was rubbed against something and abraded. I would simply apply some antibiotic ointment and monitor.