Bit of a tricky one that, you would assume that if a specialist is testing injectors and they say they're ok, then they must be.

I think you need a better test plan.
Are you able to graph the fuel pressure and drive it til you hear knock, it could be that the pressure is just too high and causing the knock, does your live data show specified and actual fuel pressure values?

<>

Does the knock and black smoke happen at 10550 psi or 6000psi? What rpm and boost pressure are you at this time?