Hello,
Darkening of the penis and scrotal sacs occur during adolescence due to the hormonal changes associated with testosterone at that age.It is normal.If however there is patchy discoloration ,then medical causes have to be probed.
Hope it helps.Take care and pls do keep me posted on how you are doing or if you have any additional doubts.Kind regards.
Hormonal changes during adolescence give black colour.It is to remain.