I have not used one of these camera....first the digital backs...yes...you can...the PhaseOne LightPhase which is a 6mp back that has to be tethered to use...would be fine with the camera...I have one for sale for $450.00....second...the black focusing screen..is your DOF stuck on...

if that is not the case...get yourself a cheap LED ring light to put at the front of the lens....you can pick-up a cheapy from ebay for about 40USD...this will give light at the front of the lens...which in turn should make the prisim less...dark..just the nature of split-prisms and low light situations...
The type of back you want is a Hasselblad V digital back....
Hope this helps some....Tony C.