7075, when hard anodized, is naturally colored a dark grey to almost black.....
this is what the color is on most AR receivers, although some are painted.
I have the acid and the power supply, just haven't sat down an done some sample
coupons to get the process sorted out. Actually, just a sulphuric acid solution in
an ice bath, and apply a low voltage DC charge to the parts. Probably want a sparger
to mix it well, so a ss tube wiht tiny holes set into a SS container, with a low pressure air source
for the constant mixing. Voltage is around 15 volts, current density numbers are published, may
need to adjust voltage to get the proper current density. My power supply has variable DC
voltage and an output ammeter. After the anodizing process, rinse and then boil
for 1/2 hour in clear water to harden the surface. That's pretty much it.
Bill