By alloy 308, do you mean the former SAE 308 which is now known as 380? I have some experience with hardcoated 380 components and you can't build much of a film.Plus, cosmetically it looks poor. With 6061, you can readily build .001" or more hardcoat film, whereas with 380 you are lucky if you get .0003". You could possibly dye the anodic film black if you want a black finish. As for wear resistance, you'll just have to test the hardcoated 380 to determine if is acceptable. Another alternative you might consider if you are looking at die castings would be alloy 390. This is a hypereutic with enhanced wear resitance due to large primary silicon particles.For a black finish, you would probably want to go with a chromate conversion, such as Alodine or Irridite, dyed black.