Comets and most asteroids are extremely dark objets. They typically reflect only a few percent of the incoming light. I will discuss the nature of dark Solar System small bodies based on ground-based observations, in situ observationand laboratory measurements. I will discuss in particular the case of C-type near-Earth asteroids, which are being targetted by two sample-return missions and how they might (or not) be representative of the main-belt population. Finally, I will present the case of D-type asteroids, and how they relate to comets and wheter (or not) we have in hand samples from this population.