What Causes Black Patches On The Vagina?

I was recently with a woman who had black patches or large black freckles in her virgina. I have never seen anything like that before. I have searched the net and cant find out what it is. Is it contagious, a STD or just a medical condition? Are there any long term or short term health problems for me. The woman is 46, I dont know her medical history or if she is still menstruating.