ISX CM 570 BLACK SMOKE

[Wiseman]

With your experience, you shouldn't have any problem to check for intake and exhaust leaks.
If that checks good , pool both sides off the turbo cartridge and inspect turbine and compressor wheel for integrity. After all you didn't mention what's the boost under full load .
What's the EGT's now and before. That's essential information to diagnose the problem.
I gues your turbo is HX-55 !? They are almost indestructible as long as there's no CAC and exhaust leaks . Driving with such will rise the EGT's extremely high and will burn/melt the outermost edges off turbine in short time , leading to getting worse by day. I bett , this didn't happen overnight.
Driving by boost and EGT is must !!! Teach your driver . If truck doesn't have boost and pyro, install them ASAP .