Welcome to the forum,
The film is curious but may have been from the mixture of items that you stripped. It could be anything from an aluminum compound to a film coating that is found on some items. This is a good example of why I recommend processing like items in a single batch. Mixing and matching can lead to many possible unknowns.
Do you have a photo of the mystery black substance?
The blue color in your foils is likely copper that was not removed by your wash process. When washing it is not enough to rinse with HCl and then water, you must wash the foils until the rinse water is 100% free of color and the HCl does not darken when added to the foils. Think of it like scrubbing a dirty pan. Just a quick rinse with soap is not always good enough to remove the grime from the pan. You need to wash it until it is 100% clean.
You can add solid copper to cement the gold out and start over. The cemented gold will be very dark brown or possibly even black, a sign that it is contaminated. Rinse the cemented powder three times in hot water, a boil in HCl, a few rinses in water, and finally red heat incineration (don't melt it!). Finally, dissolve it again and your solution should be golden yellow to red-orange depending on the amount of dissolved gold in the solution.
How much material did you process?
Steve
