It's in Paradise Valley, most parking garages have great lighting for such a thing but try to stay away from places with those yellow lights.

And yes, in a large group shot like this the black cars usually will get lost. It's tough to get them without overexposing the rest of the cars in the shot, especially with those bright colors. Only way to fix that is with some type of external flash or to get the darker cars under better lighting somehow than the rest.

As far as your photoshoot goes.... ideally, sunset is the perfect lighting for cars. There's about a 10-15 min window for the perfect lighting right at sunset, it will produce pics like these: