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I don't agree with the above statements. I would first say get it scanned. I think from there I would look to see if it leaving black residue around your intake anywhere. It would indicate it is sucking (non metered) air. I also recommend checking your turbo closely. Make sure your waste gate is operating properly. I would also check the EGR solenoids / connections. Beyond that the 6.5 is known for a few common problems. One being the fuel driver (mounted on the side of the injection pump), if not the pump itself. This will be indicated by the visual and codes or lack there of.

Sounds like you have clog injectors. You need to start by giving attention to your fuel system. Change fuel filters, Add injector cleaner to the fuel tank. The kind that takes water out. Diesel is bad about building water in the tank.

I'm not a chevy tech so I cant be allot of help here but I can tell you that black smoke is caused by one of two problems. Its either a egr issue are you have an over-fuel condition

Turbocharger is in need of replacement