I have a 2007 GMC Sierra 3500 less than 10000 miles has black colored sludge I can see in my coolant system causing problems with apparently so much blockage of this that causing it to overheat ?
The most common cause for this would be an internal leak in the radiator, meaning the transmission fluid is contaminating the coolant and causing the sludge. This would be considered a manufacturer defect and should be covered under warranty. Having the dealer replace the radiator and flush system would be the fix.
Another thing that can cause this would be if you/someone put mixed the wrong type of coolant with what is already in it (if they added green coolant) this will cause the coolant to turn brownish/black and sludge up. complete flush and fill of system would be the fix.