Blair Valley, like Yaqui Pass, is in the Anza-Borrego State Park. Camping is allowed on any dry plant-free spot next to one of the dirt roads. Fires are only allowed in metal containers. There are a pair of vault toilets and many such spots just off of the county highway. I did not get any cellular reception. While the place I camped was dry when I parked there, it rained that night.
Marshal South and his family lived on "Ghost Mountain" for sixteen years starting in 1932. I took the hike to the ruins of his house. One of the photos is looking down on the valley from near this house, my motorhome can be seen in the parking area for the trail. There are also a couple of Indian sites with trails to them. The park visitors center has a film they will show about this family on request.
Note that I have added a link to Passport America on the sidebar. If you stay in RV parks, you can probably save money by joining, and I get a commission for each person that joins through the link.