Why do most science classrooms use similar black-covered desks?
I found an article that explains the reason quite clearly: the black top is a resin laminate that is chemical and stain resistant.  

Not all bench-tops are black: there are alternate materials that are white or metallic. Presumably those substances have different resistances for different purposes; for example, some of the white benches I saw while searching were anti-static materials for electronics work.  For anything with biological and chemical substances, however, it seems that the black resin laminates are often the preferred material.