It would not be healthy for the fish to be in darkness like that for that long, and could even cause problems with their eyes when the black plastic is lifted and they are exposed to sunlight again. Think getting up in the middle of the night and turning a light on suddenly -- only "nighttime" for the fish would be MONTHS, not hours. Definitely go with the clear plastic.

Definitely NOT black plastic, that would act like a cover over a compost heap, by cutting off the light and building up the heat inside, it will kill off your plants and make the pond water sour.

If you can purchase a "white" PVC tarpaulin, they last much longer than regular polyethylene sheeting.
That type of translucent covering acts like a green house and keeps the area a little warmer, plus it allows light in.
Don't go for any dark coloured covering.
"Horton"

The clear plastic permits good light for the plants to photosynthesise and stay alive. Water quality would deteriorate if the plants die off all at one time.

The black plastic would last some years longer, ultra violet is slower to make it brittle with carbon stopping ultra violet penetration. Downside, in the absence of good light submerged aquatic plants and algae would all die off at the same time, for lack of light

It could be a succesfull compromise on a segmented frame to have both clear and black plastic as a winter cover, if the supporting structure allows you to take advantage of different sheets of plastic, placing clear plastic where it catches the best of Winter sun and black plastic on the shaded side

There is clear (or at least translucent) plastic available that is UV resistant in rolls at Menards or similar stores. There is even a completely clear plastic sold at Walmart in the sewing section that is resistant though it is much more expensive and not as wide. They both do the work well. I have used both and like the Walmart stuff better. It is easier to work with. Sandy