Since I managed to get lost trying to find the Black Rock Mountain Bike Area in Falls City (despite being more or less a local), I thought others might benefit from a clear guide. This follows the route from Salem.
Take Highway 22 west out of Salem:
Continue on Highway 22:
Stay in the left lane and turn left toward Dallas:
Continue into Dallas. At the light, turn left onto Highway 223, Kings Valley Highway (Wal-Mart is ahead, on the right):
Proceed through downtown Dallas. Watch your speed, it's all a 20mph zone, and the police are quite bored here. Turn right onto Washington:
Santiam Bicycle is about 1/8 mile on the left, should you need a Black Rock map
Keep left onto Highway 223, Kings Valley Highway:
Turn right onto Falls City Road:
Entering Falls City:
At this point, begin looking for the blue signs pointing the way to Black Rock.
After passing the town grocery store, keep right at the intersection, onto Black Rock Road:
Keep left at the fork:
The road will turn fairly rough, with loose gravel. The traction is better than you think, I tested it thoroughly... ;-)
Turn right at Socialist Valley Road:
Socialist Valley Road leads immediately to the private Tapawingo campground:
Outside this gate, on the right, is parking. Get on your bicycle, continue past this gate, and follow the road to the orange gate (you can't miss it) and the trailhead will be on the right. Maintain slow speeds until the trailhead is reached, since this is private property which is graciously being allowed use by bicyclists.