Sendcutsend customer service said that in general their different sheet metal products were conductive enough for a low power computer fan, but they couldn't really tell me what the best material was unfortunately.
Hum. The higher the resistance of the contact, the less power that will go to the fan. I've never seen much about anodised copper, but the surface of anodised aluminium is not considered to be very electrically conductive. It is usual to machine off a little area where you put grounding studs for that reason.
Will the inductor work for blue steel even if it changes?
It will anneal it fine and let you bend it, I'm not certain what exact materials you're talking about though, I'd expect "blue steel" might be some kind of spring steel? But it would help if you mentioned the grades of the metals.
If it is spring steel or something similar, you can anneal it and then harden it as much as you like by heating / quenching, heating, cooling. My memory is terrible, so best find a guide for that and verify the type of steel you're using is appropriate for heat treating.
The induction heater will anneal anything that has been heat treated or work hardened though. And it is a fun thing to have. You can use it to blue screws, like on old fashioned pretty clocks.