People will tell you how it was the opposite X years ago boys used to wear pink and girls used to wear blue. This is actually not quite true. Children were dressed in mostly white for a very long time because christening gowns are white. For example for the 1700's here's Marie Antoinette's youngest daughter Sophie Beatrice and youngest son Louis XVII. Here are two girls in white in the early 1800's.

In 1855 the New York Times reported on a "baby show" put on by P.T. Barnum, exhibiting "one hundred and odd babies" dressed in pinks, blues, and other colors seemingly without regard to gender.

The earliest reference to blue and pink I can think of is in the book Little Women (written in 1869), one of the main character had twins of a boy and a girl. Her sister Amy said she tied a blue ribbon to the boy and the pink ribbon to the girl after "the French fashion".

For a few decades afterward some publications suggest that pink should be for boys and blue for girls. Ladies Home Journal in 1918 said "The reason is that pink, being a more decided and stronger color, is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl."

There is no consensus on the appropriate color for each gender to be honest until around and after WWII. The Nazis used pink triangles to identify homosexuals . In 1948 the then Princess Elizabeth set up the nursery with blue ribbons for the future Prince Charles. Supposedly at this time people began to buy more and more ready-made baby products and manufacturers began to push for gender-specify merchandise to boost sale.