The Black Cat ring modulator was based on the rare and expensive Maestro ring modulator from the early 1970s. It is a complex effect, which gives weird metallic and sometimes non-musical sounds by using mathematical rather than harmonic blending of tones. Technically, it takes an input signal and modulates it with a sinewave oscillator to produce a sum and difference signal which is re-mixed with the original signal. The best sounds are made by attaching an external control pedal for the modulation frequency. Most standard voltage controllers (VC) with a stereo 1/4" plug on the end and just a potentiometer inside should work. This allows sweeping the modulation frequency as you play, or finding the "sweet spot" to the key you are playing in, for more harmonic sounds. You can play the same notes on the guitar and have it sound different by "playing" the pedal.
Here is the Manual