Hi;
I have noticed that in photos the X-15 isn't actually black! Or, if it is, it looks like it has a slight graphite sheen to it! I believe that's a result of surface heating on the Ablative coating the bird is covered in.
I think that if I was to build another I would paint it, Alclad Gloss Black with Graphite Semi - Gloss and mostly clear over spray. This bird, like the U-, has an odd coloring for sure