I buy a concentrated liquid carnuba wax, mix it with water at about 10 to 1 in a spray bottle, and it works great on my black car. Saves me a lot of $$, since an once of wax can do the whole car. Wax is like any other liquid in that even when diluted, it still leaves a layer of wax on the surface of the car. It's the wax that shines the car, not the thickness of the layer, like when the wax is applied undiluted. This is why Meguiars came out with the spray detailer. The water acts like a wash, and the wax adds the shine.
I went 3 years between a complete wash, since all I ever needed to do was to remove surface dust. If I got caught in the rain, I would just spray the car with water, and then apply the spray wax. JMO, but I think that cars can be overwaxed, which to me is an un-necessary expense.
Of course, since I drive my car, I am not going for a show car finish. Shiny looks just the same when going down the road.
