Anti-halation backing
No image visible? It may have the anti-halation on the base, making the film completely opaque (black). It cannot be removed once the film has dried. This will wipe or bleach off while the film is still wet. Try wiping it off with a Chux. You will need to wipe hard. Be as careful as you can not to scratch the emulsion. Alternatively, bleach it off. Pour out 1-2 cups of bleach into a container (eg ice cream carton). Immerse film in bleach. Small black flakes will appear very quickly. As soon as they do, pull film out of bleach and wash under running water. You will still need to wipe it. Timing is critical as the bleach will also be acting on your emulsion.