I use the fuji film quite a lot and the positive base is white if the chemicals haven't spread. I recently puled one out and the chemical pod ripped off before it opened and spread on the film.

black would be chemicals but no exposure, half a frame does suggest there is something "in the way".

Ursula,

do you have anyway of taking the half used cartridge out - darkroom, darkbag - and then store it in (I use another polaroid or polaroid back) so you can check the camera again? if the problem is so consistent and the same every time you will probably need to do this anyway, it doesn't sound like it's a problem with the film