Silver hair was relatively rare among the Sindar. Only Thingol's family seems to have had it (even Cirdan was one of Thingol's relatives).

Blond hair is attested among the Sindar, believe it or not. Thranduil, at least, is described as having golden hair in THE HOBBIT. And many people have wondered who/what that golden-haired elf was in "Lothlorien" who helped the Fellowship cross the stream in the forest.

As for the Beornings, I guess that since they were Northmen the MERP people decided it would be okay to infer some blonds among them. I don't actually object to the idea myself, as the history of the Northmen is quite convoluted. I infer (without any real textual evidence that I can think of) that the Beornings represented a strong Beorian strain among the peoples of the Vales of Anduin. THE PEOPLES OF MIDDLE-EARTH says that most of the Edain of Eriador were Beorians and most of the Edain of Wilderland were Marachians. Nonetheless, even in Beleriand there were golden-haired people among the Beorians and dark-haired people among the Marachians.

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Parma Endorion: Essays on Middle-earth, Revised Edition