In all the videos of Portuguese egg tart molds I see on Youtube, they all appear to be fully black. Carbon steel doesn't seem to be as dark as that (although it could simply be lighting), and I'm not too sure what sort of reaction would cause metal molds to darken like that.
I first thought that it was aluminum oxide forming on the molds, but from what I can find online that would seem to taint any tart with black residue, so that doesn't appear to be it.