Blow off tube left black marks

[petismaximus]

Tried using a blow off tube for the first time with it going into a bucket filled with water and sanitizer but noticed after a few days there are black marks on the bucket above the water line. It hasn't cleaned off very well and I can't seem to figure out what would cause that. We used an extra fermenting bucket we had laying around so if it can't get cleaned off completely I don't want to keep using it if it's bad. Any idea what happened?

 

[Gameface]

Are you talking about a ring left behind from the krausen?

The yeast often leave a deposit just above the water line. Perfectly normal. Don't try to mess with it until after you transfer the beer and are ready to clean the bucket. Oxy clean and warm water will take care of it.