Final answer:
The black spots on the x-ray film in a colony hybridization experiment represent the colonies on the filter that contain the desired cDNA sequence.
Step-by-step explanation:
The black spots seen on the x-ray film after completing the colony hybridization experiment represent the presence of colonies on the filter that contain a recombinant plasmid with the target cDNA sequence. The experiment involves rinsing the filters to remove un-bound radioactive oligomer probe and placing them on x-ray film. Black spots form on the film due to radioactive exposure, creating an autoradiograph of the filter. These black spots correspond to the colonies on the filter that contain the desired cDNA sequence.