Hey,
Maestro is the only dark mule we have. He doesn't show any of his cross. He had a faint cross when he was born but when he turned to black bay, he lost it.
We did have a black mule born last year but he was 2 months early. We had him for 25 hours but he just wasn't ready (that one hurt!).
He was a coal black boy with a silver muzzle. You could see his cross. It was even darker than he was. He would have been an outstanding mule had he survived.
My other mules have all been light colored and they don't show their crosses either. Don't really know what to tell you on this one.......Sorry!