The blind cap is the same celluloid as the barrel - just that with this angle and rotation, you are seeing the dark brown part of the pattern, so it looks like a black blind cap.
Interesting - and thanks. Now, do you know if the nib would have been branded Aurea? The one that I put in the pen is a repaired/reworked Duofold nib.
Aurea branded nibs exist for sure. Here is one: