Black particles in coolant?

[Mr 5 0]

Coupe5oh:

Could be random paint flakes from the new radiator.

Adding coolant isn't abnormal if it's only a bit at a time. Keep a sharp eye for leaks, though, just in case.
Water pump vents are a common place, hoses of course and the dreaded head gasket leak with the antifreeze in the oil are all places to check regularly if you're suspicious.

If the heater core is leaking, you'll know it by the smell and the wetness on the floor.