I think Diesel has used Black Gold to address a direction that they couldn't really do under the Diesel tent. Diesel has a positive, flippant, cheeky thing going on and Black Gold is a more severe, "forward and directional" undertaking.
They're not really addressing young/old - they're just catering to a wider range of tastes by splitting the Diesel bucket into two separate piles that wouldn't really make sense if they were intermingled.
Will be very interesting to see where Bruno Collin is going to take the brand. From the sound of things, Diesel really will be changing gears. The past few seasons have been really off the mark, so I see this news as a great thing.