Why many people from North Blue are blonde?

Let list people from North Blue..

1.Doflamingo
2.Rocinante
3.Machvise
4.Jora
5.Dellinger
6.Sanji
7.Bellamy

there is also connection between these characters Doffy and ROcinante were both from nobles,same with Sabo who is also a blonde noble,and Sanji might also be a prince of nobles..

do you know why?
share your thought..