The antifreeze appears black in color and consistency of a black mud?

Hi,

If they checked for a head gasket issue, I question if they are performing the flush to clean the system. What I would suggest is this. Drain the entire system and flush it.

Here is a link that shows in general how it's done:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

To bleed the system, do the following. After filling the cooling system, start the engine and allow it to reach operating temperature with the radiator cap removed. Air in the system is bled through the radiator cap opening. Add coolant as necessary to bring to the proper level, then install radiator cap and check the coolant level in recovery reservoir.

Now here is my biggest concern. This vehicle came with GM DexCool antifreeze/coolant. This product can not be mixed with traditional coolants. If it is, basically what you described is what results (looks like mud). That is why I'm recommending to completely drain the cooling system and flushing it.

When you refill the system, use a universal coolant that is compatible with DexCool. I suggest this in the event some of the old coolant is still in the vehicle.

I hope this helps. Let me know if you have other questions.

Take care,

Joe