It wouldn't have been for IMC Training. The safety pilot has to be able to see outside, all around.
The only way you can blackout the cockpit for the pilot flying is to use the old blue/amber system. That is where amber screens are placed over the transparencies and the flying pilot wears blue goggles. Blue and amber makes black; the instrument panel is a bit blue but the safety pilot still has the world in amber.
That went out of fashion in the RAF in the fifties.