Next time you watch it Marga, pay close attention during the ballet dream sequence about half way through. A dark-haired (short cropped like a modified close-fitting bob) woman looking exactly like Ms. Charisse comes out and seductively puts her leg around Gene Kelly and dances with him. SHE, I believe, is wearing a long black dress with a very high slit up the side. Her legs, face and height look like Cyd's, as well as her dancing style. It's a very short piece and the back drop is a bar. It ends with a mob-like looking guy walking out tossing a coin with THE GIRL.
If that is not Cyd Charrise (uncredited, I know) then please tell me who it is.
I know it's not Nina Foch, who is blonde and an actress, not a dancer.