You only need a thin layer of the stuff and a fresh CLEAN microfibre or you will get stain spots that require a lot of buffing.
It does work well on black and is very noticable but doesnt give a metallic flake 'pop' as white diamond would.
Suited to more solid colours imo rather than metallics.
I was impressed with the black hole on my metallic black mk3 and the flake pop is bloody brilliant(UltrafinaIII,Megs205,Poorboys black hole,jetseal 109 and DJ Supernatural)
But ive heard a lot of different people saying use white diamond for metallic pop so i will be giving this a go next time.