Hi, happy to help you on your health concern on part of HCM!
In the present case of arm pit swelling or lump, if I were your consulting physician, I would just think of only one and one possibility and that is a sebaceous cyst
or epidermal inclusion cyst!
As per your description, it is a benign harmless lesion and never turns malignant but dark discoloration might be there due to silent recurrent bleeding within its cavity! Anyways, surgical removal is the definitive cure for these!
In any case this could not be cancer...
Take caution not to get injured there as this might invite infection.
If you are still worried, please get examined by a surgeon or get some FNAC like cytological examination or revert back to us.
Hope you got your answer. Please feel free to write to us if any more queries.
Wishing you the best health!