Shiny black
Generally, dull black
Sodium is shiny, but it reacts very quickly with oxygen in the air to form lithium oxide, which is a dull black color.
It is in fact the best radiator. Dull black is the best absorber and emitter. Then shiny black, followed by white, then silver. While the best reflector goes the other way around (silver's the best, then white, followed by shiny black, then dull black)
Shiny
dull
dull
dull
dull
dull
It is shiny.
Shiny