What kind of wrap did you use?
I used something rather cheap, found by google search. (50 meter roll for some €1/meter) It was 5 cm wide and 2mm strong but I do not remember the shop. I wrapped with overlap, so isolation was 4mm. The cheap stuff lost tension after some month and by removing it I found the blackend pipes.
I am pretty sure any other proper wrap will cause the same effect.
If you are going for the black surface only, you could ask a gunsmith for the chemical for blackening gun barrels. That causes a chemical process of controlled corrosion of the very top layer of metal surface to seal it. "Brünierung" is the German word for the process.