In the last days of the Republic, Anakin Skywalker, the Hero With No Fear died. He was reborn as Darth Vader, Dark Lord of the Sith.
In the first days of the Empire, Padme Amidala died. Her cause of death was threefold: the collapse of the Republic she had served so loyally; the betrayal of her husband; and the loss of her unborn children.
She too was reborn, in the figure of the Black Widow.
In the first years of the Empire, wronged women of the galaxy flocked to her banner. Those who had lost husbands, siblings, parents, children... They allied themselves with the fledgling Rebellion, training and fighting alongside them.
The Empire's soldiers feared the Black Widows. These women had nothing to lose, and nothing to live for but revenge. And they would rather die than be captured.
They never wore black, oddly enough. Their uniform was white, pristine... the better to oppose the Empire's darkness.