If you took a rider who could handle the black run at llandegla and took them to an alpine/scottish/other black run and set them off they’d be seriously underestimating the technical difficulty, therein lies the problem.
Has this problem ever actually arisen? Surely people are capable of realising that not all black runs are created equal, what happens to the poor confused lambs when they venture onto bridleways that don’t have a grading?
There are 4 trails at llandegla, hence blue/green/red/black. They could call them all blue, but that would get confusing.