All under the hood heater boxes and fan boxes under the fender in 61,62 & 63 were a satin to gloss black color.
The heater box under the dash in 61 & 62 was satin to gloss black( 10 original cars I did).
I have a factory original low mile mint complete 63 heater and the under dash heater box is a dark gray.
My 57 Chevy has a under dash heater box that was and is now gray.
I bought Eastwood gray paint for it in Spray Gray color which is a medium gray color.
Eastwood may have a darker gray.