Call her mother or the maid of honor. Guests don't invite guests to weddings. Saying you would like to bring someone puts the couple in an awkward position. The only time it is okay to ask to bring a date is if the bride doesn't know you are engaged or living with someone. Black is too funereal for a daytime wedding.
Black prints are acceptable any time of day.
White, once reserved for the bride, has become more acceptable, but it remains the subject of controversy. The editors of Bride's find white suits and tailored dresses acceptable but suggest accenting them with bright or pastel blouses, belts, shoes, purses, or scarves. The idea is not to look like you're competing with the bride. The etiquette books say no, and it's almost always a good rule to follow. It used to be any time after the departure of the bride and groom.