Hi Robbie,
The Dark Peak is a part of the Derbyshire Pennines that consists mainly of Millstone Grit a darkish stone. The White Peak is the part of the Pennines which is mainly Limestone, a white stone.
I use Dark Peak as a misnomer as I'm actually in the white peak... I am at the gateway to the Derbyshire dales. Matlock crich etc. If you saw Peak practice I live in the village it was filmed in.
Cheers Rhino