Precautionary Discharging Capacitor went Terribly Wrong | SMPS
Shouldn't the capacitor go bad if a huge bang had occurred followed by soot on the PCB?


It appears that you short-circuited high-voltage, high-capacity capacitors. These store a lot of energy and you discharged it all in an instant so you generated a very high current and generated an arc due to poor contact. The soot on the PCB was generated externally to the capacitor. Your capacitor is probably fine.

In future you should discharge the capacitor in a controlled fashion by using a resistor. Work out the power that would be dissipated in the resistor if it was permanently wired across the capacitor while powered. (You could probably use half the calculated wattage for a brief discharge.) Make up some insulated probes attached to the resistor.