I’ve attempted it once before using advice from a couple guys at FSM and the result was crappy. If anybody has a link to a good tutorial on how to do that I’ll appreciate it. It’s actually harder than it seems at first glance.
I’ve had fairly good results with painting the jackets a “super-dark” grey of my own mix, with black in the shadow areas, then a lighter grey highlight. I then cover it with a clear satin varnish (NOT semi-gloss)… The satin gives it just the right amount of “sheen” and looks “leather”, to me anyway…

Not the best photo, but you can kinda see it… I did these 1/48th guys in black to scale down the brown leather…
Similar answer, don’t start with too dark a base color, you’ll have no where to go for shadows.
I use Vallejo acrylics.
For black leather I will begin with black mixed with flesh color to lighten. For highlights and wear mix in a little orange, for shadows add more black. Brown leather is the same idea, but begins with a brownish/red base then add darker browns for shadows, lighten with orange or even a little yellow.
Ric
I use Humbrol black enamel mixed with humbrol Flesh for a basecoat. Darken with more black for shadows.
SST
I think I tried one time mixing in a hint of a real dark purple or blue with the grey before going over it was a black wash and it came out looking pretty good. It was almost like a fine wash of the dark color if I remember correctly, but real dark it was almost a black purple/blue. And as stated, go for the satin.
Just something new to try, I experimented around with it so I’m not sure what exactly I did.
I start with a near black base blend a deep maroon oil overall, then purple black tones into the low spots. Then a subtle blend of powder blue on the high points. My colors are derived from an FSM article on painting black from a few years ago.

This link was given to me to help me with painting flesh, but it had leather.
A simple technique is to use a wash of black ink (assuming you’re painting black leather!). Paint the item of clothing as you would normally, but accentuate any highlights by making them lighter in colour, then simply use black ink as a wash over what you’ve painted.
I suggest you might want to experiment with this technique on an old figure. The best effect might require the ink to be thinned (with water), depending on the brand you’re using.
Al
I’m gona “bump” this topic again for a moment…
I looked at the link Smeagol recommended (Aces of Iron–great link!!) and really like that effect for worn leather, but I am wanting to paint a relatively new issued German black leather long winter coat (btw…I’m using Testors/Model Master Enamel paints). Does anybody recommend using a gray or white-ish color for highlights on a black leather coat??
Well, Ill toss in my .02
I have zero experience in painting figures (but Im fixin to get some!), but it would seem to me if you are seeking to replicate newer black leather, then you could start with a base of flat black, use some dark grey or white for the highlights, blend well and you should be good to go.
Also, once the paint is dry, rub your fingers over it—gives the leather a “sheen” to it from the oils in your skin…