What will make 1973 Dart rear package shelf black again?

the replacements are the same material particle board or similar to it. I would can spray it. The paint absorbs into the material pretty quick and will dull naturally. A few even coats should give you the desired effect. A cool temp and shade should produce the best results.
It should be and look fine mine was replaced only because it was missing altogether. Otherwise I would have gone this route without a doubt.