I know the answer, but I was wondering if you did too.
Without googling, my belief was that pink for girls came from the Nazis’ assigning it to gays (pink triangles?), making it “feminine”, and that beforehand pink had been a boys’ colour! Not sure about blue - possibly related to the pink thing.
It was my understanding that blue was a protective colour, so you would protect the boys from bad spirits.
Pink was just the cheapest colour available. Because, well, girls aren’t that important.
I would like to know…so tell us.
ChimneySweep is right. Pink being feminine came from Germany and their use of the pink triangle. I cannot explain blue for boys, but the conjecture is that blue is the color of a military uniform.
Here is the link: .http://answers.google.com/answers/threadview?id=238733
That varies Extremely.
Little Boy Blue is a nursery rhyme with probable origins in the Middle Ages. Little Boy Blue was a hayward by profession.
Rhyme:
Little boy blue, come blow your horn.
The sheep’s in the meadow, the cow’s in the corn.
Where’s the boy who looks after the sheep?
Under the haystack, fast asleep.
Will you wake him? No, not I
For if I do he’s sure to cry.
Perhaps not relevant, but certainly pre-dates the influences mentioned so far.
Because boys have the power. Blue starts with B, and so does boy. They liked this, and took blue to represent them. Pink was the only thing left, so that’s what girls got. If they had the power, they could have brought green into the whole situation and stuck us with red.
BECAUSE:
The little girl on the left looks positively miserable.
Pink is softer relaxing color.
Blue is a dominant stronger color
Guys that wear pink I don’t know… I am trying to picture a Hell’s Angel biker in pink. Rather frightening picture. 
If you read the article I posted it said people pre-WWII saw red as the masculine color and boys wore pink as a symbol of being almost a man.
Peter Norton.
Blue is the colour of Jim Morrison’s voice, the closest auditive equivalent of depth and gravity. It is the colour of the sea, patroned by Poseidon and traversed by feline wits such as Ulysses and Aeneas. Blue is also the colour of Lapis Lazuli, which is something Yeats wrote when he was bored. A masculine undertone indeed.
Pink is lighter, more graceful, usually flowery-like. It is more like the voice of David Gilmour at a helium-balloon party and it suggests optimism and other positive things. I have a pink t-shirt in my wardrobe. I used to have some pink underwear as well, as a consequence of mixing it up with a red t-shirt in the washing machine.
Link is not working.
What was, does not mean it is. ChimneySweep and you are right about the why and how, but now it is for different reasons the colors signify gender. If a boy today wore a pink suit, that boy is going to have some problems.
Xunian was correct.
Blue was a magic color of protection in the British Isles. The Scotts and other tribes would paint themselves blue, as in Braveheart, because they thought it magic. Girls didn’t rate for that type of protection.