This is a very long shot . . but I wonder if you contact Nordicware www.nordicware.com and explain what is happening, they might be able to provide an explanation and also some help on a remedy.they do have a use/care section on their site, but you're probably doing that already.http://www.nordicware.com/resources/use-care-instructionsPlease let's know how you get on. I'd be very curious to know what to do in a situation as you've described.
Like this one Rosie? I inherited this one from my MIL who probably got it from her Mother who was a great cook. My MIL probably never used it as she was not a cook. No black powder on mine though and if yours is as old as mine it would have been well seasoned and used by those before you. Have you tried just scrubbing it with a steel wool, I'm sure you can't damage it at all. I'm sure Andie will be able to give you the correct answer as she has many antique items in her collection.
Maybe different vintages Rosie but yours looks like cast iron to me. If no-one can give you ideas of cleaning the marks off yours, I'd place a circle of baking paper in the bottom before filling them up.CC my ex MIL had these gem trays too and she made the best jelly cakes. You get a perfectly round cake with these trays.
AndieSenji what would we do without you and your knowledge? Wonderful. Hope it works for you Rosie.Gretch