One thing with black cars you have to really pay attention to is refelction. in the second photos your feet and tripod are clearly visable in the shot. and the reflection off the surface almost makes the car look dirty. Flash in my opinion helps but reflections become even more apparent because it picks up the lightscource. I problaby would have put some light on these to give it more pop.
I'm still learning myself, but here's what I've come up with.
1. Car HAS to be clean. Dark cars show dirt so easily.
2. Don't get too contrasty in editing. I used to lose a lot of details by having too much contrast. It has a cool effect but with dark cars you cant go too far with it.
3. Shoot in RAW and try shooting on the lighter side. Capture the details and then if it's too light, you can adjust it in post-processing.
4. Use a Circular Polarizing Filter. It reduces glare and reflections which tend to be worse with black cars especially.
I personally like these photos, especially the first. Yes, you can see your legs and tripod in the second, but that's a minor mistake. The car looks a little dirty to me. You could've used a CP Filter and more light (especially flashes or other man-made light so you can control what you are lighting).