Suggest Treatment For Dark Spots On The Elbow

Hi, a few marks ago, marks started appearing in the crook of my elbow on both arms. It is not itchy, nor have the marks spread. What has happened is that they have gotten progressively darker. My dermatologist says that he doesnt think that it is any infection. Any ideas on why these marks have appeared? The dermatologist suggested that I apply a cortisone cream for seborrheic dermatisis. It didnt have any effect in removing the marks, but stopping the cream for a few days resulted in the marks going very dark very quickly......Any ideas? thanks

I am a 27 year old female (1.5m, 52kg, mild seborrheic dermatisis on face)