From the description it is some what clear that the bump was actually a collection of old blood. It is called as an hematoma
and is nothing to worry about. You might have hurt your self unknowingly where in the blood got collected and became black with passage of time. You should become concerned, if there are a multiple of such bumps in your other part of the body, If not a single bump should not be a matter of concern and you can relax.
Hope I am able to solve your concerns,
Thanks and hope you get well soon.