Is it okay for my mom to wear a black dress to my afternoon wedding?

There are two dresses at David's Bridal that my mom loves and which look really good on her. One of them is in black, and we know it's okay to wear black to an evening wedding because of the time of day. However, my mom is really concerned about wearing black to my afternoon, summer wedding, even though it's... show more There are two dresses at David's Bridal that my mom loves and which look really good on her. One of them is in black, and we know it's okay to wear black to an evening wedding because of the time of day. However, my mom is really concerned about wearing black to my afternoon, summer wedding, even though it's becoming popular to do so. Here are links to the two dresses she likes:

http://www.davidsbridal.com/mother_of_bride_detail.jsp?stid=3736&prodgroup=59 (in the blue)
http://www.davidsbridal.com/mother_of_bride_detail.jsp?stid=3370&prodgroup=59 (the black one)

I told her that I don't care what she wears, as long as she feels comfortable in it. She likes both dresses, my sister and I like the black one on her best. What's the protocol regarding wearing a black dress to an afternoon wedding, if you're the MOB?
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