At the outset, it doesn’t sound like they’re connected at all.
Black dresses for women are popular, dresses and the color black probably mean different things to different people subconsciously - it depends of thier personal experience and feelings about both.
If the same person had both dreams, maybe there’d be a connection if thier intuition told them the dress and it’s color were significant. But that isn’t the case.
Except… if it’s a message to you because you’re the other common denominator. In which case, what does a black dress mean to you personally? I.e., what other memories and feelings do you have about women wearing black dresses?
Try following that train of thought in a musing meditative state, maybe google ‘black dress’ and see where it leads you. Pay attention to what comes up and be open to signs, and the meaning may come to you that way.
It also might not show up for a few months - write the dream down before you forget the details. It will help you remember so you can look back at it later.