The maximum smoke value emissions test for diesel vehicles varies depending on the age and type of vehicle. Assuming your car is turbocharged the limit is 3.0m-1 if registered before 1st July 2008, but 1.5m-1 if registered after this date, so if your car is in the latter category it has fallen some way short of passing. You may wish to try an oil and filter change as well as an additive to improve the result, but it may be that a more through inspection is required to determine the cause.