[Saraphim]

I slept through most of Biology class. Hey, it was first period.
But I did copy my notes from my friend, so I know the basics of the basics of the basics of genetics.

First of all, I know that the term 'half-elf' is kind of a blanket term to describe beings with both human and elven blood. But technichally, Arwen was 'three-fourths-elf' (three of her grandparents were elves; only one human).

Elrond passed on dark hair, which overrides light hair something like 80% of the time. Arwen's 'midnight sheen' or whatever is just a literary description of her beauty. She may have had a sort of light sheen to her dark hair, but that's because she was the Undomiel and all of that, and supposed to be beutiful.

Most likely, Arwen would have carried her children for 9 months. Because a) He children were mortal, and b) so was she, technically. They would also, most likely, have had dark hair, because the 80% thing I mentioned earlier would be up to something like 99% with Aragorn's dark hair in the equation.