The pit side of the argument sure seems to agree with the half-dozen unrestored, unmolested, original '68-'69 B-bodies I've owned, insofar as the blackout was on undersides of the rad supports but not the upwards-facing areas.
If it wasn't a pit, perhaps the sprayer just reached in from the front and happened to angle the gun upwards, like if the hood was closed. But I don't recall ever seeing black overspray on the underside of the hoods.
It seems logical that the sprayer wouldn't be far away, for the sake of coating efficiency.
Either way, I'm not losing sleep over it; but I'd sure like to see an assembly-line photo so the question/debate can be settled.