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Dyes and different colorings are often mixed into materials. These are usually a small fraction of the total material volume/mass and aren't painted on so don't have a uniform surface coat but some manage to block all light or all light from a certain wavelength.
How does this work? Why can widely/randomly spaces molecules block light?
If you are referring to "all light," it's important to clarify that you mean visible light, as other types of light, such as radio waves and x-rays, would most likely pass through your mixture. When dyes and colorings are added to a material, it becomes increasingly opaque, obstructing the passage of photons in the visible light frequency. Additionally, the transparency of a material can depend on polarization. Polarized glasses, for example, appear transparent until photons with a polarization perpendicular to the glasses are attempted to be transmitted. In such cases, those photons are absorbed, similar to how an opaque material would absorb them.
