If you paint yourself you've understood the issue. Not only black is difficult to paint but it's also a pain to create contrast between the different elements of the same color.
So I had to use different techniques for each black parts to simulate different type of textures.
Black metal for the gun and the protection on tibias.
Extreme white highlights with black armor washes for the leather on the boots, the gloves and the belt.
Soft grey highlights for the quilted knee protection and the fabric of the shirt.
It's not very visible on the final pics but I've achieved to obtain a different rendering of black for each part. ouf!
No, Cobra wasn't a so easy painting as I though, but the I'm happy with the result. He looks like Snake, good point.
Bob Hauk: We'd make one hell of a team, Snake!
Snake Plissken: The name's Plissken!