I found 25% sited in several web sites without a reference. But, I think that number was for the US population. A more detailed Google search turned up a 2% number for the world population, but that was just an estimate.
I found another site that gives a 9% for the US population, which is what I think is close to the truth. They also pointed out that 70% of hair coloring products are for blond hair, so the number of people who look blond is higher than those who are naturally blond.