I can understand your concerns,first I thought it could be a lipoma
which is the collection of fatty tissue in a site and is a benign growth and can be left alone if your triglycerides and cholesterol
are in normal range,but when you said that there is a black spot on the surface then it raised my concerns . A picture of the site from a closed angle would have been very helpful but it's not there so I cannot comment any further.
I would strongly suggest to see a dermatologist
who has a surgical interests or if not possible them a general surgeon
would be I think a good option to evaluate the lump.
I hope this discussion would be of help to you.
Best of luck.