Think about it:
What color do we gravitate towards when "dressing-up"? BLACK.
And when we are going out, what is our main goal? FUN.
I've established this direct correlation:
BLACK = FUN.
The idea of having a good time is consistant throughout our history of party going and cocktail attire, though the LBD in itself has shifted slightly. I still consider a simple black dress with an elegant string of pearls to be the ultimate, sophisticated and stylist look (way to go grandma!) but I've also seen accepted the LBD to come in several variations. All of which are equally fabulous.
Traditional with a Twist.
Textured material or contrasting on white.
Making Black Bright.
Using sequins, metallics and studs.
Playing with Prints.
Black as an accent to create definition.