Not sure if Dr Chris Fellows should do this as a research project. The main thing is that I am conjecturing that what makes comet's nucleus' surfaces black is crude oil (in a solid gel form) rather than agglomerated soot.
I was not sure what it was about crude oil that made it black, but apparently neither is anybody else. I had it in my head that it was the "impurities" that made it black, ie possibly soot particles within the crude oil or something like that, because distilled oil is usually *not* black. Soft enough to absorb micrometeoric impacts, but solid enough that it keeps its form, especially jets, from collapsing in on themselves on a comet.
I am afraid that at present, no one can answer this deceptively simple question. Even the most basic structural properties of the asphaltenes and their aggregates remain conjectural. I believe that optical properties reflect collective electronic transitions in the PAH aggregates, but this is nothing more than an educated guess.
Why are oil and tar black?