The custom is based on an English poem:
Something old, something new
Something borrowed, something blue
And a silver sixpence in her shoe.
Each item refers to a good luck token for a bride. The custom is that if the bride carries all four items on her wedding day, she will have a happy marriage.
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Something old
– continuity with the bride’s family and the past
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Something new
– optimism and hope for the bride’s new life ahead
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Something borrowed
– an item from a happily married friend or family member, whose good fortune in marriage is supposed to carry over to the new bride
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Something blue
– Before the late 19th century, blue was a popular color for wedding gowns, as evidenced in proverbs like, “Marry in blue, lover be true.”
(via http://wiki.weddingbee.com/Something_old,_something_new,_something_borrowed,_something_blue)