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.
I grabbed a picture from this video around 3:08, but I found another example here too:
Best Answer
That looks to me to be seasoned carbon steel for both the individual cups and the trays.
When first seasoning cast iron or carbon steel, it can look orange or brown, but as you cook it repeatedly (or to a higher temperature), it will turn black, as in the videos that you linked to.