Batwister -
In your Hasselblad body, behind the mirror, there are a pair of doors/light baffles that open when the shutter fires. The firing sequence on a Hassy is as follows:
-depress shutter button
-shutter closes
-lens stops down to working aperture
-mirror pops up
-baffles open
-shutter opens
-shutter closes
-baffles close
Winding the film advance then triggers the mirror to return, and re-cocks the shutter, opening the lens to maximum aperture for viewing.
If those baffles are not operating properly, they can fail to get out of the way of the light, leading to partially exposed frames. This is what I'm suggesting you check first, by dry-firing the camera without a film back on, to observe their actuation. I don't remember if the problem with them occurs with short or long exposures - I THINK the problem is with exposures greater than 1 second because they don't stay open, but I could have it ass-backwards (it's been 15 years since I had the problem, on a body I no longer own). The suggestion to check the lens aperture diaphragm was a secondary thought, because you're shooting with the lens almost to minimum aperture anyway.
Are you calculating reciprocity with these exposures? Although color negative film requires much less exposure compensation than black-and-white, it still requires one stop additional exposure for times over 1 second. Depending on your film, you may have to add two stops past 30 seconds, or some other time - be sure to check the films' data sheet.