Greetings!
I've found that painting black can be a challenge, but it isn't impossible to do well while still maintaining some good detail and not taking a ton of time either.
First, let me give you an example of one of my deathwatch marines to illustrate the technique.
This is a
WIP picture of my now finished Librarian in terminator armor.
As you can see, I've gone in and dry-brushed a nice blue-grey shade. Any grey will work, honestly, but I found a blue-grey gave the armor a nice blued/cold effect that I really liked. If you go with other colors/tints to your drybrush it'll give the black a variety of shades. The more neutral greys will give the black a deeper, less cold feeling, depending on how light or heavy you want the highlights to look.
here's another example on some regular power armor:
My technique is simple. I dry brushed the grey shade over the hard edges, paying special attention to places there was a 90 degree corner. I then went back and anywhere there was a smooth panel (knee pad, the upper leg, and lower leg for example) I used a fine detail brush and 'filled in' those areas with black if I got some paint on that panel/space while dry brushing, leaving a nice thin line of grey along the edges. I then went back and used the edge of my brush to edge highlight a few very specific points with the same blue-grey shade, such as the 90 degree angles, so they were brightest.
This gives a nice depth to the black and doesn't take an exorbitant amount of time. You can use a second, brighter shade of grey as the second edge highlight to really make the hard edges stand out nicely as well. Try a few different colors and see what works/looks best to you.
Hopefully this was somewhat helpful. and if you have any questions feel free to ask.
Take it easy for now.
-Red__Thirst-