As long as the guest is comfortable and their attire doesn't detract from the wedding, a black dress is fine. Although I can think of one time when it wasn't fine. I was a bridesmaid in my best friend's wedding. The ceremony was outdoors in a beautiful gazebo. It had rained a bit that morning, but the sun came out just in time. It was a bit chilly, but not enough that we needed coats or sweaters over our dresses. The bride's mother had her own ideas. While getting ready at her house, I noticed that the bride's mom had a strange-looking dress hung on a doorway in the hall. It was dark blue, made of a thin jersey-knit material with a little iron-on rainbow patch on the front. It looked like a well-worn, casual dress from the 70s that came out of the back of her closet. I didn't think much of it. Surely her mother-of-the-bride dress was in the bedroom!
Nope! Mom comes out wearing the dark blue dress. Then she proceeded to wrap herself up in a long, black cloak complete with a hood that nearly covered her entire face. My friend, the bride, didn't say a word. I always knew her mom was weird, but this was next-level weird. We got out to the gazebo which was in Mom's condo complex. There were pretty pink flowers. Us bridesmaid's were in dusty rose-colored dresses, as was the groom's mom. And here comes the mother-of-the-bride, marching up the aisle in her black cloak. She at least pulled it back from her face for the pictures afterward, but she kept the hood up. It was quite odd!
I kid, but my family has told me that I should stop wearing black to weddings. 
