"C" was only an example I believe. However,if the signals given are in Concert C, Bb people would always have to transpose the 2 step difference in their heads to move to the new key.

The greater part of my music career was playing in a band. Sax players and trumpet players and bass players wanted the hands signs to be given from a concert key of C major. They transposed the rest themselves.

How confusing t would be if we were giving hand signals based on the different instruments ( Bb Eb) in the band.

In all my 45+ years of playing with other musicians I have never seen a musician use Up for flat keys and Down for sharp keys.

Just saying, John C.