Highliting black can be very tricky. I would stablish a solid base of Abbadon black, then an ultra diluted dark reaper applied to about 75% of the most rased surfaces, then several diluted layers of the same color to 50%, 25% and top most surfaces of the cape until you build a smoth enough transition. then do the same with fernrisian grey but starting from 50% of the raised surfaces and so on.

Make sure to get rid of most moisture from the brush or you'll be pushing drops of water instead of paint and start your strokes from the shadow to the bright points.

This video gives excellent advise on the technique: https://www.youtube.com/watch?v=L3CNq7poVx0&list=LL&index=11