Tuning How are pcp stainless tanks blued/colored black?

3000 PSI Tanks are usually aluminum and powder coated 

I drill holes in 2 or 3 weekly when they have reached there rehydro date. (own a paintball pro shop and 5 playing fields plus a small gun shop)

Ok thanks. I thought it's stainless steel because it's durable yet it won't get blued by a standard bluing compounds. Rubbing blue remover didn't do anything[normal steel gives that light gray color], and rubbing muriatic acid onto the tank created quite a dark layer[it should also be light gray i think for normal steel], so i thought it was stainless steel. Apparently you might be right -it can be aluminum, but im not sure, it doesnt feel like aluminum. 

And by the tank i don't mean scuba tank -i mean the PCP reservoir under barrel!
 
Hmmm i wonder how hard would be to anodize the tank if its aluminum? Im pretty sure its aluminum because sanding with sandpaper was extremely difficult[as it should be -it's aluminum oxide we are sanding here, with aluminum oxide paper :/ ]. Weird that anodized surface would discolor from sweaty hand mark, thats why it fooled me and got me thinking its bluing. 

Anyone anodized in home their PCP reservoir?