Depends on local custom. In some places, wearing a black dress to a wedding is a complete no-no. In others, it is becoming the norm for fancy weddings.

But I don't think women are ever required to wear black. You might go with a dark color if you don't want to stand out in case others are wearing black -- I attended a similarly fancy wedding and wore a simple navy blue floor-length dress, and I felt completely at ease. Most of the other women were also in floor-length dresses of various colors, but some of the younger women (I was a younger woman at the time!) wore black cocktail dresses, and I did not feel out of place next to them the way I might have in a pastel or brighter color.