You have the right intuition that light will interact with different structures based on their scale. Specifically, if a structure is much smaller than the light wavelength, the light will effectively smooth it out and ignore it. This is exactly the case of the scratches on the metal surface.
However, this will also happen for light passing around particles; if the particles are much smaller than its wavelength, it will ignore it or effectively interact with the "smoothed out" bulk. If, on the other hand, the size of the particle is comparable to the light's wavelength, it will interact, most likely absorbing the light. It turns out there are even companies that produce golden nanoparticles of various sizes to achieve different colors:
The point of these colors is that light of roughly the wavelength of the size of the particles gets absorbed, light of shorter wavelength gets diffused, and light of longer wavelengths sees the "smoothed out surface" and gets either neatly reflected or passes through.
Now to your example of black nanoparticles. The previous example corresponds to round nanoparticles, that is, particles that do not have any substructure. However, if you make a particle of a more complex shape involving more than one length-scale, such as the spike-balls below, they will absorb light of a broader range of wavelengths and thus a more "black-like" color.
From what I have read, a golden nanoparticle solution will degrade over time and the particles will get stuck together (aggregate). Various combinations of the aggregate particles provide various obstacles for light of various wavelengths and thus absorption of a broad spectrum. Broad-spectrum absorption (in the optical) is exactly what we call "being black".