I don't have a -48VDC power supply to test it with, but as far as I know +48VDC has the same effect. Firmly grabbing a lead with both hands results in absolutely nothing happening.
Grab the same leads of +48VDC, but switch your hands, left lead to your right hand, right lead to your left hand. You will get -48VDC.
Hit us with those stats on people killed by touching 48volt systems. Compare that with people killed by AC at home.