The Black Card is kinda oxymoronic, when you use this card you are definitely in the black some way or another usually with having way too much cheese.
They are simply a status thing, as if you are rich enough to get an invitation you aren't really in need of it. It's basically an accessory like a Rolex watch. It looks good to pull it out and impress people with.
Subscribe in a reader 
American Express Black Card 
Linear Mode
