Lump on the rear shoulder turning black.

A lump on the rear of my wife's right shoulder (on the back right below the right shoulder) which started off as a small pimple a year ago has now grown to be a little bigger than a pea. Earlier it was not causing any irritation or pain and we were hoping that it would probably go away but just from the past 1 week, it has begun to ache a little. The outer skin of this cyst is black and it has now started to cause a bit of concern. Please have a look and please tell us what it is and how it can be treated? Some of the great doctors here might have seen this before or studied about it. So we are hoping to find out if it is something that is just nothing or something that needs to be taken care of immediately. Many thanks in advance