The “Black” I don’t think is officially that but was suggested. It’s just the old Satan’s Grotto trail which is very short and no longer in any way challenging after having had some work on it to bridge the bog and smooth the roots. In my Swinley grading it’s almost a red (with the red being blue and the blue being green 😛 ). Or at least would be by Welsh standards.
Other hidden gems… there are none 😉 (as far as the general public is concerned. “Local trails for local people, there’s nothing for you here”).
That said. Being more serious, follow Trail Team Swinley on FB and Twitter for progress on trail developments. In the main the plans going forward are I believe related to developing the Freeride and DH areas, which have been given the go ahead last I heard.