black smoke?

my peugeot expert 20hdi 2002 is chucking out alot of black smoke the turbo, injectors, air flow sensors and timing have all been changed the van is giving no fault codes it drives and sounds fine apart from the odd surge that causes more black smoke the garage is now saying it is the mid sction of the exhast is blocked could this be the reason thanks for any help

Barry shrimpton
September 2007