What is the longest black ski run in the world?

The longest black piste in in the whole world is the Sarenne piste in Alpe d'Huez.

It's on the piste map (from the Alpe d'Huez home page) stated as 19 km but on the Alpe d'Huez home page it says 16 km so the actual length is a bit unclear.

It can be noted that last part of the piste (which is several kilometers) after coming down to the bottom of the Sarenne gorge is more transport than black.