Shawn,
The last time I hiked out of Black Rock was in October or November many years ago. I came in from 395 and the weather was turning bad as I drove in past Kennedy Meadows and by the time I turned off to drive up to Black Rock Gap it was snowing. As far as landmarks go, the Black Rock Ranger Station is right on the corner of the turn-off to Black Rock Gap and it is shown on the Forest Service Map for that area. The road dead-ends at the trail head. You can't miss it. Call the Kernville Ranger station to see if the BRRS is still open. There is a snow gate that is closed after the first big snow.
To help specify the time, there was a bear hunt going on and it was between deer seasons or the first season was just ending. Also there was a ranger still on duty at the Black Rock station prior to it closing down for the winter. That is about as close as I can pinpoint the time of year. This had to have been at least 20 years ago. By the way I was driving a sedan. If you have a 4-wheeler and chains I’d give it a go. However check with the ranger to make sure they don’t lock the gate behind you.