The black stuff is a combination of things. Grease and oil from snowcats, fireplace ash, and who knows what else.
Riding Stratton last Thurs and Fri, it didn't get too slushy on the lower mt. til around 1PM or so. I did crash several times in the slush, but my Coiler RC 180 seemed to handle it well if I wasn't too aggressive.
'Tis going fast, alas