I have a 66 chevelle I picked up last year with a 283 and 52'000 miles. When I got it I drained the coolant and it was dirty which I expected. I put a recored radiator and ran a overflow tank in it. So I drive it and check the coolant and it has a black liquid in it. I check the overflow tank and it has the same black substance. It's a thick black substance and I cant figure out what it is. To clean the tank out I have to use the garden hose at full pressure to wash it out. I did a coolant pressure check and that was fine and it doesnt seem to be oil because it doesn't smell like it. I changed the coolant a couple of times and it comes back a week later. It leaves a thick black liquid in the overflow tank and you kind see the black in the radiator. I was thinking maybe the car wasn't flushed in a long time and the car was drove on short drips that the coolant didn't warm up enough and left this black stuff in the coolant system. If anybody has any ideas what it might be it would be appreciated.

It sure sounds like oil...does it seem to float or sink to the bottom of the coolant? Also, do you see a rainbow-like sheen across the top of the coolant surface...may help to have it in a wide bowl of some sort, or get some of your black liquid and put it in clear water. If you see the rainbow, it is most likely some type of hydrocarbon.

I bought a $400 beater car years ago...went to change the coolant and about a quart of black goo (what I took to be oil) puked out of the radiator. I said a few bad words and went ahead and flushed and refilled the radiator. The goo seemed to "come back" one more time...most likely residue, then it never came back for 40,000 miles?!? So I dunno if someone just got crazy and thought the radiator was an oil tank or what.

Another check for hydrocarbon is to look closely at the radiator hoses...generally hoses compatible with antifreeze will swell and soften in the presence of hydrocarbons.

Good Luck

It might be radiator sealant , Bars Leaks brand
sealant is greyish black pellets suspended in a black oilly looking liquid.Nasty stuff but it does the job if your in a pinch.

Yep, could have been "junk" in the heater core or hoses that's 'moved' with the new coolant. Backflush whole system with heater in the ON position till its clear, then drain and add correct coolant. You'll probably have to remove and 'hand clean' the overflow can.