I have a couple black Canon 7 rangefinders, and they aren't easy to fake. If it's a real black camera, it will be black enamel over brass, any chrome plating underneath means it's a fake.
Most black Canon 7 cameras come with a two-tone silver and black rewind knob, and a silver and black frame line adjuster, while a few are entirely black. As far as I have been able to tell, there are no particular serial numbers to identify colors.