Black time is, depending on of course the urgency of the appointment 1-4 hours.
Other factors such as family, work, and random meetings between acquaintances can add an extra half an hour.
People that are usually on "black time" are over popular people for whatever reason, or a person with no backbone.
Tanya- When is your friend coming by to hangout?
Sean- He didn't give me a time. He said he would be here soon though. Then again you know how he is always on "black time."