Crud in coolant

I have a 99 Suburban with 101,000 miles on the 5.7 350. This past weekend I decided to swap out the coolant and was dismayed to be draining black liquid from the radiator. It was not thick, did not look like mayonaise, but was quite slippery. I refilled the system and noticed that the fluid in the coolant recovery tank was black within a week. The Suburban has the combined oil/water/transmission cooler radiator. Anyone ever experience and internal leak of one of these things giving rise to my symptoms? I should mention that I changed the oil at the same time and did not notice anything weird about the used oil coming out of the engine....

Oli in the coolant can also be a sign of a blown head gasket, although it's usually coolant that's found in the oil. A leak down test would be necessary to eliminate the head gasket. You may also want to pressure your cooling system for leaks to determine if there's a leak in that system.