Participant Photo: Tufted Titmouse with unique mark beside eye

A pair of Tufted Titmice nested in our umbrella pole by our pool in April 2021. After noticing them bringing nesting material, I started to watch them. After a little bit, I noticed that the female had a black mark beside her right eye, (I knew she was a female because I watched her and her mate copulate). I know that it wasn’t just a stain on her feathers because throughout the whole month that they were here, the mark never faded or went away. I named her Queen because from certain points the mark looks like a crown.
Does anyone know what caused this? Is it just a unique birthmark or some genetic thing? Could it be a very small case of melanism?
Any feedback is appreciated!