What Causes A Dark Purple Spot On The Upper Back?

Hi, accouple of years back I started with a small dark purplish spot on the center of my upper bacK (above bra strap), as yrs gone by it extended pretty big(about 2 hands put together side by side)it stopped extending now. about 4-5 yrs ago i went to a dermatologist and she Rx me some bleaching cream but sure didnt work. It does't itch or anything it ust bothers me cause i cant wear what i want in the summer. what can it be?