Black Mark on Comb

[Ettore.fin]

This has recently appeared on one of my chickens, anything to worry about?

 

[Marie2020]

I had one of my girls with a smaller round shape, looks similar. I can't tell you what it is but the moment I saw this black spot I smothered it in oil, it was gone the next day. Would be interested to find out what it is. Hopefully someone here will know. Really impressed with all I've read here.