Two things are at play, the lignin (light absorbing) is emphasized and the cellulose is burnt away (light reflecting), and it seems this combined pretty unique

Two things are at play, the lignin (light absorbing) is emphasized and the cellulose is burnt away (light reflecting), and it seems this combined pretty unique lattice system (they use a similar synthetic system when producing regular ultrablack paint but the natural one appears to be more complex) create the blackness.