I work in a dental office that uses cerec and most of them have a black collar around margin, why is that?

There are many possibilities....  During the bonding process, if bleeding was an issue, the chemicals used to control moisture must be removed completely or black lines appear a few weeks later.  If the restoration wasn't properly bonded, saliva and bacteria can leak in and discolor.  If decay was present and not completely removed, it can discolor.Among other things.It is most likely a bonding protocol issue.

It sounds like there is a serious technique problem in your office.  My partner hand I have been doing CEREC restorations for over 20 years beginning with CEREC one.   All restorations have a margin where the restoration and the tooth meet.   This is filled with cement or bonded cements and the cementation is critical.If the area is not completely dried and isolated you can get leakage that looks black from bacterial growth.  Another cause is bleeding if the blood gets incorporated into cement.A poor fitting crown (should not be a problem with CEREC) can have larger cement gaps that may show some discoloration but should never be black.We have done thousands and thousands of Cerecs and though my partner and I don't always use the same technique we always see good results.

It may be poor design with large gaps that have been sealed with resin cement.  Resin cement can later stain depending on coffee/smoking habits thus leaving a dark brown line. It happens also veneers with poor margins.