then it probably should be marked beyond double black.
I don't want to get too off topic, but that is exactly what double black can mean, IMO. There are a lot of those slopes, especially if you're unfamiliar or don't know how to self arrest. Slope ratings are not universal, and generally (though not elways) double black will be the most difficult rating.
A lot of places will add "Expert terrain" or "Experts only" or "Cliffs" to indicate real "don't fall" areas, but if you're going into double black terrain, you should already be careful and realize there could be consequences and know what you're doing and getting into.
The only place I can think of that's decided to use something beyond double black is Big Sky, which has marked some runs triple black. The problem with that is the relativity of slope ratings being only applicable at that resort - what's a triple black, why this run and not that one, etc. I think current ratings and markings are plenty.
Meanwhile, Palisades and a few other places go the other way, and just mark all black terrain as single black, with some other "experts" or "cliffs" signs for particular places/runs.