First of all, that's absolutely gorgeous dcurvy!!!
I can see why you're having some trouble. That's a really dark background to work with and it can cause trouble. I know as I've had trouble with this as well. I've had some luck with an artist's white chalk pencil on darks in the past. They're a little heavier and "chalkier" than some of the quilting pencils and markers. You can usually find that type of chalk pencil at any craft store. Hope you find something that will work for you. It's going to be the perfect design for that border!