welcome to HCM.
Blackish spot does not mean gangrene
. It could be small blood clot, which would gradually disappear.
As long as the child does not have any discomfort or symptoms, there is absolutely nothing to worry about.
To be on the safe side, it is better that you consult the surgeon who performed the surgery and let him examine the throat and do the needful.
Wishing your child all the best.