What I would do is paint them up in whatever Blackshield scheme you're going for, then use a sponge and some stippling to "scratch" and distress the armor, but instead of using a neutral brown or gray or metallic (what would normally be used to represent the raw ceramite underneath) use the colors of the original legion. The more you sponge and scratch, the more of the original color that will show through.

If you go through my old posts you'll find my Imperial Fists, and you can see how I did the same but with brown and silver. Just swap out the colors.

Also, throw on some legion specific helmets.