A couple of months back, I went down there to conquer both Black Mt and Mt Anslie.
Here's what my Garmin 305 GPS recorded.
BLACK MT
From the Cnr at the bottom.
Up = 244 metres
Dist = 2.70
Av. Grade = 10.3 (there are a few (short) bits where you either level out or go slightly down so sums may seem strange)
Max Grade = 15.6
My Av speed 11.1 (not bad for first time AND I only go down to 39 / 23

)
MT ANSLIE
From the Cnr at the bottom
UP = 222 metres
Dist = 2.94
Av Grade = 8.9 (there are a few (more than Black Mt) bits where you either level out or go slightly down so sums may seem strange)
Max Grade = 14.5
My av speed = 11.7 (rode straight over after Black Mt - but this has more flat at the bottom to increase the av.)