Old car engine oil is used in metalwork to give a black finish, the only problem is this is done by heating
said metal to cherry red. Easier to achieve with steel and iron than ali. Still might be worth experimenting with a blow torch. How big is said HS?
To be honest I'd just spray it black with bog standard spray paint, doubt it'll ever get hot enough to damage the paint. Think of a car roof in summer, gets hopping on one leg waving your hand about hot.
If that don't appeal here's -
https://www.ebay.co.uk/sch/i.html?_f...paint&_sacat=0 various high temp paints as found on ebay. I've used both the barrel paint and SAS 59, works ok and have also used stove paint. Both the exhaust and stove paint comes off eventually, but then both hospital hot. I've had my burner glowing orange, not blacking can survive that.
Andy.