Has anyone found a product that can make black paint that has been oxidized to a grey haze, black again?

Parts stores and probably Walmart have a product called Color Back. It will revitalize the color, but nothing will truly bring it back. The pigments have changed in the paint.

That detail shop either doesn't know what they're doing or the clear coat on the hood has pealed away.

Take it to a reputable body shop and get a quote. If nothing else they can wet-sand the hood then polish it to a gloss again. Or, it may have to be repainted. They'll be able to tell you.

thats probably clear coat,but you can use meguiars fine cut cleaner and swirl remover.it is made for dark colors

Just one; black paint.