Progress so far. I tried the Brake Cleaner on a scrap of fabric, and it didn't damage the fabric, so I felt safe trying it on this dress. I put a cardboard tube over the mark, and sprayed the brake cleaner into it (so that it wouldn't go anywhere it shouldn't). I repeated this several times, but no joy. The stain was still there, albeit a tad less black!
So, to plan B. I cut out an applique shape this morning, undid the back seam, and matched up, as best I could. I didn't want to stitch, as I thought that would make it even more noticeable. Applique cut roughly, I ironed the applique sticky stuff onto it. I peeled the paper off the back, and jiggled it into the best position I could, and ironed it on.
If I can obtain some acetone, and it removes the mark, I can peel it off. If not, it's not a bad fix.
Thank you all for your very helpful suggestions. I knew you lot would help me sort it out!
What do you think?
Jessie