Developed X-ray film can reduce light, so in a basic sense it may work as a DIY neutral density filter. Historically, blackened film has been used as a dense light-reducing filter. However, it is not a good substitute for a proper photographic ND filter.

The main issue is image quality: X-ray film usually has emulsion on both sides, giving it high density from metallic silver buildup, but also more turbidity and reduced optical clarity. That means it may soften the image, lower contrast, and introduce unevenness or color/tonal shifts rather than acting like a truly neutral, optically flat ND filter.

So: yes, you can experiment with it, but expect compromises. If you already have a standard ND or polarizing filter, those are likely better choices. One suggested at-home workaround is a pinhole overlay made from foil over a filter, though that changes the imaging method rather than simply acting like an ND.

For best long-exposure results, a real photographic ND filter is still the better option.