How many hours on the engine? Does the engine run up to WOT RPM once it gets going, or does it not reach the specified WOT RPM's? Has it always done this or is this a new issue? When were the hull and props last cleaned? Volvo turbos are notorious for short life spans (as well as ridiculous cost), but you nearly always see this as a loss of WOT RPM's rather than just black smoke at low rpms.
Most decent common rail/electronic diesels will self compensate rather than over fuel and black smoke if there are issues with boost or blocked air intakes etc. I don't know the D12 system at all, and I don't trust Volvo engineering a lot, but your mechanics suspicion of bad injectors would probably have been my first pick also if the electronic brain they use is half decent in design.