Small dispersed particles of metals are often black, while a solid object of the same material would be some shade of silver.
The most notable example is the use of metallic silver as the black color in classic black and white photography. It is chemically deposited into the gelatine layer on the paper.
Another example would be working with metal in a way that creates dust from abrasion (nickel in the example case at hand) - coloring the hands dark gray. So it seems to be a general phenomenon, independent of silver.
So: What is the cause of the difference in appearance depending on particle size?
I would think it could be a process of dispersion or possibly destructive interference. Without knowing the details, that would mean the relevant particle sizes should be on the order of the wavelength of visible light? I assume - unverified - that the particles stop being black again with decreasing size at some point. The mechanism causing the silver reflecting appearance of metal is related to electrical conductivity - I expect that could be involved here too.
I will list some questions that occurred to me - they are not explicitly part of the original question:
Is an individual particle black, or is the color emergent from the properties of the ensemble?
And at which size does the change happen?
With decreasing size, does the color change from black again? And does it change to the original silver appearance? Is that dependant on the metal, or on particle size only?
Is the transition gradually with size, or abrupt?
Is there anything else that influences the change, so that particles of the same size can be silver or black depending on some other condition?
As the black color seems to depend on the interaction of light and opaque particles, it should be fairly independent of the metal - which properties of the metal are relevant?