my advice is the same as you'll get from blm and usfs, if it's not posted open to travel consider it closed. basically if you get out there and don't see any carsonites with an approved trail number on them then consider it a closed trial and save yourself a ticket if you should run into a ranger. also keep in mind that offroad travel is not permited inside the confines of the las vegas valley due to dust control issues. so if you're within the mountains that encircle las vegas and create this valley and traveling offroad you are out of bounds and eligable for a ticket. since i've never traveled in the blue diamond wash i can't give you a yes or no. keep the above info in mind when you go out there and make you own decision.
in the trail discription on trailraters.com of the blue diamond wash trail he mentions getting to a dam and "Once at the dam, you'll climb the hill to see some abandoned mines. For some reason, the path is blocked by gigantic rocks...". this is usually a good indication that travel beyond that point isn't wanted for some reason, private property or danger, and by continueing i'd say you are out of bounds. i'm not an expert at this stuff, just a guy that's wheeled the desert for years and has a litle experiance, so if you want to run it good luck. i'm staying away from it.