Unfortunately it's impossible to not have a bit of light shine through black (or any color) in our film or transparent media's.
All colors, including black, will be illuminated by the lights behind it. Translucent material is white, with black printed on it, but it remains translucent, it doesn't become solid or opaque. We also can't print on solid black material as the print would not show up.
If you need solid black or solid colors on film media, you'd need to work with a special sign cutting supplier as this is a special use case that inkjet printing cannot solve.
You can see samples of backlit film printing under the SAMPLES tab in Backlit Film in MEDIA, or click here.