US here
I go to lots of weddings, and many, many women wear black. It's the most common color, in fact, and not at all always accessorized with color. And it looks just fine.
Now, I'm talking about evening weddings. During the day, you would see much less black.
Because the point of the "rule," to the extent there is one, is simply not to look like you are going to a funeral, or like the evil fairy come to curse the marriage. A black party dress with party shoes, bag, and jewelry doesn't look at all like either. During the day, a solid black suit or dress or pants outfit might; I'd choose something else or add colored accessories.
Maybe that's why it's different in the UK, where, I believe I have read here, wedding ceremonies must take place during the day. UK readers: if there is a separate party at night, do women wear black then?
As far as not wearing the same color as the attendants, some people do seem to care about that, but there is no etiquette rule about it. I can't imagine it's even an issue other than possibly for the bride's female contemporaries who don't want to look like they are bridesmaid wannabes. Most guests aren't even going to have any idea (or have even thought about, let alone cared) what the attendants' colors are anyway before they get there.
As for mothers -- rules are really meaningless here! The bride's vision is what matters, so try to wear whatever the bride asks you to, rules or no rules. At my son's wedding, the bride asked her mother and me not to wear black, and she preferred we find something in green, which is what her attendants wore. I found a green dress (different shade from the attendants); her mom didn't and wore a taupe color that harmonized nicely. My daughter didn't care what color anyone, including the attendants wore, except for white, and I wore a black dress with magenta accents. So there you are -- one wedding wearing the same color as the attendants, and one wearing black!