02 Duramax nearly 53000 miles, excessive black smoke when pushing accelerator down, like when you are passing somebody on the freeway.

Just started in the last few thousand miles, took truck to dealer ship and had rail psi checked and injectors checked on the computer dealership said numbers look very good. Any ideas?

I'm guessing no mods. Check your air filter.

Could also be a boost leak. Check the blue hoses and clamps on the turbo, intercooler and intake.