Raw aluminum can oxidize and get some nasty crud on it if its left raw for a long time (i.e. played with, gets dirty, left wet, etc.)
If thats all youre needing annod, your best bet is to polish it yourself to get it glossy, then ask a local anno company if you can sneak it into a run of black when they do it. Most anno companies do a lot charge for one color where its a base charge for one tank - so, if theyre running a tank of black and they have room for your feedneck, theyll just throw it in for you.
One thing to consider though, those feednecks are pressed in the body, so if you anno it before you press it in, you might scratch through the anno. Not sure if that matters since its not going to be coming out, but might be something to think about.
Vintage matrix CNC milling and parts thread Here