Maybe you can get a better idea looking at this map.
Lower Blue Lake Campground, CA - N38.61735° W119.92139°
The spot I'm talking about is at the second picture in this thread if I remember right.
I'm thinking the sign is right where the road turns north for Upper and sort of south for Twin Lake. In front of it is a parking area. ???
Oh Yah! It shows pretty good in this one. I don't know if that helps any.
Lower Blue Lake Campground, CA - N38.61021° W119.92376°