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"Thus, the blue color in Kirlian photography appears to be due to the ionization of air molecules above the
film plane.. "

Explanation of the phenomenon

During the Kirlian process, a few electrons are first produced between the electrodes; these are accelerated by the field and they ionize air molecules, leading to and exponential growth in the number of electrons and positive ions.
The electrons are attracted to the (+) annode and the positive ions move slowly to the (-) cathode. When the ions in the air reach a critical density, they strongly attract the electrons. Recombinations then accur and photons of light are generated.
The clusters of positive ions become brightly luminous and travel at high speeds. Both positive and negative streamers move between the electrodes, and discrete balls of light move in various directions.

The normal color of the streamers is a bright blue since the radiation from excited nitrogen molecules predominate.
This electrical process produces enough light to expose the film and results in a photograph of the type under discussion.
Thus, the blue color in Kirlian photography appears to be due to the ionization of air molecules above the film plane.
This ionization is a direct result of energization by the electrical corona emitted from the fingertips, for example.