Nasty coolant. Bad head gasket?

[79vette]

Truck is a 1988 cab and chassis with a 5.7l, fairly new to me. I changed the oil a couple weeks ago and it was clean and clear, no evidence of coolant. No overheating, no visible bubbles in the radiator or overflow bottle with the engine running. The truck does seem to use a little coolant, but it is leaking from the radiator cap and I assumed that was the only problem. I changed the cap with a new one today and we'll see if the leaking stops.

But the radiator is full of nasty black water like I have never seen before, but it is not "goopy". Its a low viscosity fluid like water/coolant should be. The exhaust does blow white smoke for 2-3 min after startup. The white smoke lasts a bit longer than I've seen from other SBCs in the past, but I thought it was just condensation until the engine warmed up since it did eventually go away.

Is a compression test my next step? Flush the coolant and see if it turns black again?
If anything, this looks like combustion gas leaking into the cooling system through a head gasket leak or head/block crack. But if that were the case, I would expect water to be spraying out of the radiator with the cap off and engine on...