
The Fellowship of the Ring, The Great River, Verse 54 (if I counted right) describes Frodo looking up at Legolas standing tall above him, as the Elf gazed into the night, looking for a mark to shoot at. From this point of view Legolas' head was described as: "dark, crowned with sharp white stars that glittered in the black pools of the sky behind."
As it was night however, folks disagree whether or not this is definitive with respect to hair colour.
Another path some take is to employ Appendix F, which generally describes the Eldar as tall, fair of skin, grey-eyed "though their locks were dark, save in the golden house of Finarfin" — this thinking appears to be that if a given Elf's hair is not definitively described, he might as well be popped into the general description set out by this statement, no matter that in The Hobbit, for example, the golden-haired Elven King of Mirkwood turned out to be Thranduil (but King Thranduil is, again, definitively described in any case).
As a small aside, there's a footnote added to later editions stating that these remarks in Appendix F "applied only to the Noldor" — and indeed they had, in certain draft text at least — but for the final, published version, Tolkien had this apply to the Eldar, and in the 1960s, he himself didn't revise the description in question; and indeed, in The War of The Jewels for example, the Sindar are generally described as resembling the Noldor in hair colour.
So far, in any case, I myself prefer to picture Legolas as dark-haired.
And of course, there's a certain cat in a coat that has a lot of dark fur,
but that has nothing to do with it. Okay. . . that has little to do with it.
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