In the Lord of the Rings, no mention is made of Legolas' hair colour.
I think at the end of the Appendixes Tolkien stated (to the effect) " all the Eldar were dark except the Golden house of Finarfin"
Does this imply he had dark hair?
The evidence is fairly neatly summed up here: http://www.tolkiensociety.org/faq01.html#legolas
However, in Quendi and Eldar (HoME 11) we read the following:
In general the Sindar appear to have very closely resembled the Exiles, being dark-haired, strong and tall, but lithe. Indeed they could hardly be told apart except by their eyes; for the eyes of all the Elves that had dwelt in Aman impressed those of Middle-earth by their piercing brightness.
But this should be considered indicative rather than conclusive, especially bearing in mind the points made in the Tolkien Society link, above.
In summary: we don't know and there are valid reasons to argue in favour of either fair or dark, so take your pick and form your own internal image.