Just Wondering.....What are your thoughts on the mother of either bride or groom wearing black to the wedding?

My future Mother in Law is wearing a black tea length dress with like white/black chiffon on the top and the oversized sleeves. Now we have a great relationship, I already call her mom since my fiance and I have been together for 8 1/2 years and are getting married next Saturday!! YAY : ) But she told some of the ladies that she works with what she was wearing and they all told her that she should ask me what I think of it, cuz they umed I would have a big problem with it. I don't have ANY problem with her wearing that, actually I love the dress and it's so cute on her and encourage her to wear it. I was just wondering what you guys think of this and how would you react if they showed up in black on your wedding day? Have a Great Day!! Thanks : )