Legion of the Damned are on the easier side of painting black (which generally isn't easy). All you really need to do with LoD is make sure you keep the edging on the power armour nice and crisp and it'll be great. I'd recommend 4-5 layers feathered together ranging from really dark gray to a medium or lighter gray. How light you go is a function of how much pop you want on that part of the fig. If you want to add battle damage and weathering that's a whole other thing of course
Another option, and on that I like a lot, is actually to use shades of blue rather than gray for the edging. It still looks black, but you also get some nice colour complementing set next to flames that have a good bit or orange in the mix.