C32_Underdog

After buying a C32 with 60k miles, I did an oil change myself and a coolant flush at the dealer. The coolant flush seemed like a great deal at a great MB dealer, only $65. Five days later, I open my coolant reservoir and see black sand near the bottom of the tank on the side that leads into the radiator. I posted a pic. When I took it back to the dealer the mechanic convinced me this was normal. Now, there is even less of this sediment in the reservoir, so I assume it has entered the cooling system. Did I get screwed here? Is this going to harm the cooling system? Should I take it back and make them reflush it?

Thanks in advance