Guy Simonds joined the Army as a junior officer in the RCHA. In the Second World War, as a general officer, he habitually wore the black beret of the Canadian Armoured Corps, I always thought it was in emulation of General Montgomery, his mentor, who wore a black RAC beret as a personal trademark. Neither officer was an armoured officer (Montgomery was an infantry officer). Montgomery wore a Royal Tank Regiment badge on his.
The black beret on the face of it seems unusual. However, as a "mounted" unit, perhaps there was some indication for a black beret - perhaps the RCA Museum would be able to clarify.
After the Second World War, the arms and services adopted a 'midnight blue' beret, but the example in the auction seems to be a black one, and anyway is clearly dated 1943, long before the adoption of the midnight-blue.