That black smoke is due to the carbon dioxide gas they inject into the nozzle at the end of firing. Did you see that long arm pivot in from the side? The nozzle gets so hot they have to quench it immediately or it will cook enough to ruin the post-test measurements. On an actual launch, when the motor burns out, it is so high that there's almost no atmosphere and it's a hundred degrees or so below zero. During a ground test, though, there's lots of oxygen. Also, the inside of the motor is insulated with thick rubber, which would also continue to burn were it not for the CO2.
