The Big Bear Lake area has three black diamond trails--Dishpan Springs, Holcomb Creek and John Bull. The Holcomb Creek Trail is closed to the public because of the Butler 2 Fire in September 2007. Hopefully, it will reopen next year, IF the Forest Service can finish removing the dead hazard trees along the trail. The best time to visit is in the summer, when the daytime temps are in the 70s and low 80s.
Here's a report of a day run in which we attempted to do all three trails in a single day in July, 2006. It includes videos of the Dishpan Springs waterfall:
http://www.myjeeprocks.com/forums/showthread.php?t=6837