Alright, this is something i've tried before. You can't use bluing chemicals on plastic, very bad idea as some might fuck up the plastic.
What you can do is get a krylon flat black/camouflage black paint and do a PROPER coating on the thing. Once its all done and finished, use a very fine sandpaper very LIGHTLY on its surface to smooth out the surface of the paint, and then use very fine steel wool to further polish it.
I've done simulated 'metal' finishes for guys locally.
G&G AK I had done. The receiver was originally clear, I matched the paint to the rest of the gun via the method above.
Heres a gun I did a simulated 'pitted steel' look. Sold this to a guy on ASC, dunno where its lingering nowadays.
(Yes, thats an accordion.)
If you have any questions I'd be happy to answer!