I only remember them in a glossy black back when they were new, but I'm not an E-body guru.
I think the flat black stripes for various cars are an aftermarket invention, and though I like the look on black cars, I can't recall ever seeing them in flat black "back in the day".
Seems to me that they'd be white on dark cars, like Daytona & R/T stripes and various A-bodies were, but I can't recall.
A buddy with a black '70 HemiCuda has red hockey sticks, but I recall him saying they were done during an '80s restoration and were incorrect.
They look good, though.