Vintage on line

So I met this woman this weekend, a little strange but whatever, she claimed to have a huge collection of vintage dresses she wanted to sell. I went over to her house, and after she blabbed her life story to me I finally got a look at some of the clothing. She did have a ton of stuff, but only a few quality items. She showed me this gorgeous little black dress she had purchased in 1960, and it was adorable. I looked it over quickly and it seemed to be in pretty good shape. The zipper worked fine, there were no holes that I noticed, and it was so cute! She sold it to me for $15, which I thought was pretty good. Anyways, last night I was hanging it up in my bedroom when I noticed a huge stain! How could I not have noticed it before?! It was on the back of the dress, near the bottom, so I guess I just didn't look down that far. It's a really weird stain too, it looks almost faded...it's a strange reddish, almost burgundy color against the black. It's not obvious from far away, but up close it looks awful! I tried washing it, but it did nothing. What is this stain?! I've never seen anything like it before! Does anyone have any idea?