Sorry for asking again but is it localized or "all over"? It could actually be anti-seize if dissimilar threads are involved and it was a super quality company. As RJD indicates, the majority are from China, so who knows what that is. Something off whatever it was packed near. I'm going to guess manufacturing residue. Hope it comes off easily. Glad you are safe and well.
Aluminum radiators are available with an anodized finish (never have to paint it and adds corrosion resisitance). Many colors available, including black.
On the "pro" side of what you are doing: Aluminum heat sinks are anodized black for better heat dissipation so you are on the right track for better "performance".

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn