For darker minis in general, you'll want to start with a black undercoat, rather than white, and you'll likely want to make heavy use of washes to shade down rather than layering up. Here's an example of a marine that's free of battle damage, but still a good bit darker than your typical Blood Angel scheme. They were built up from an almost purple base, layered to a brighter red then washed back down to tie it together:
As for weathering, there are many ways to go about it. A favourite of mine for power armour is sponge weathering for realistic chipping. Take a piece of sponge or scouring pad and lightly dip it in the desired chip colour, then just dab that on the area you want to chip. It's simple, and very effective:
For powerders, just brush them on! I use chalks rather than anything fancy, just a light coat around the boots for tying the minis to the bases:
For larger chips in armour, you can take the colour you use for sponge weathering and paint it on the relevant area, then do a quick highlight of the armour colour around the edge to emphasise the chips (see the fist):
Finally, you can use washes in general to add a veneer of grime to a mini. Typhus Corrosion is a good paint to sparingly use for this, as it's formulated to look a bit gritty/oily when dry.
Hope your find some of that helpful!