This black feather from a Paradise Riflebird (Ptiloris paradiseus) still appears ultra black even when coated with gold.
This is because structural absorption contributes to the super black, velvety appearance.
These feathers have highly modified barbules – microscopic filaments on feathers – arranged in vertically tilted arrays, which significantly increase the scattering of light and thus can result in approximately only one hundredth of the reflectance of normal black feathers.
A detailed study of the feathers appears in Nature Communications.