The point is it isn't clearly defined. It's situational. Not all black people will or should say it, and some white people feel comfortable saying it and use it in situations where it doesn't offend anyone. There is no "correct" race, skin tone, culture, etc where it's always OK. There's just situations where it's OK for some people. I'm sure if you know it's OK then you know it's OK, and that's the only time it's OK. If you're wondering if it's appropriate, it probably isn't.
The point is it isn't clearly defined. It's situational. Not all black people will or should say it, and some white people feel comfortable saying it and use it in situations where it doesn't offend anyone. There is no "correct" race, skin tone, culture, etc where it's always OK. There's just situations where it's OK for some people. I'm sure if you know it's OK then you know it's OK, and that's the only time it's OK. If you're wondering if it's appropriate, it probably isn't.