For my 718 B-GTS, I went with a middle-ground between the silver and black ... the 20" Carrera Classic Wheels. My car is Night Blue Metallic so the all black wheels were just too dark but the silver to too bright for the black trim on the GTS.
I'm tellin' ya: Once you go black, you'll never go back: >
But I get it. Black wheels have to 'work' with the rest of the car, as well as be of the right design. Since I've either body-colored or blacked out almost everything else (right down to the model name, headlight washer nozzles, LED headlight housings and smoked rear taillights), I feel it works on my car -- which was part of the plan.
To the OP: For 18" wheels, I don't think black will look quite right on any wheel except possibly a very clean five-spoke design. Black effectively makes the wheels look smaller than they actually are. To my eyes, my 19s work because of the five-spoke design and a relatively short and clean rim section; a denser design wouldn't work quite as well.