According to an ancient legend, Monica, after she was widowed, asked Our Lady how she should dress after the death of her husband and son. The Blessed Virgin appeared to her dressed in a modest black dress cinched at the waist by a long leather belt. Mary took off the belt and gave it to Monica, telling her to wear it all the time and to urge those who wanted her protection to do the same.
From that day on, Monica, her son Augustine, and all who embraced his rule began to dress in this manner.