This dark patch is caused by uneven spreading of the film emulsion within the image due to a combination of roller pressure in folding type cameras, which varies from camera to camera and the viscosity of the film emulsion. There is an easy technique that helps avoiding this problem with folding type Polaroid cameras (SX-70, SLR-680, etc): Add slight downward pressure to the latch of the film compartment of your camera while the image gets ejected from the camera. This slightly increases the roller pressure and thus ensures even and complete spreading of the film emulsion.

Our camera experts in the Netherlands made a short video on this: http://blog.the-impossible-project.com/avoiding-the-notorious-undel...

Please contact us at service@the-impossible-project.com and attach some scans or digital images of your pictures showing the patches. We will then gladly help you out.