Doors? I love riddle games like these for car trips esp. when some people know the answer and a few don't. It's sooo much fun to tease the people who don't get it and try to help them figure it out.
Just so you know The Valley of the Green Glass Doors is played by saying "In the Valley of the Green Glass doors you can bring..." And then list only words with double letters like "yellow giraffes" and then list things that cannot be taken such as "brown monkeys".
Black Magic is played by pointing at multiple things and saying "This chair is not black magic. That door is not black magic. Your teeth are not black magic. His socks are not black magic" Then you point to something that IS black, and then the next thing you point to IS black magic.
So every non-black thing you point to is not black magic, and then after you point to something black, the next thing you point to is black magic.
Black magic is anything after you pointed to something black.