If my grandparents and my parents are all black-haired, how come I have brown hair?

All four of my grandparents had black hair. My parents also had black hair. Me? Brown hair. How is that possible? The only explanation I can think of is that my great-great-grandmother was from Bessarabia, a region that now belongs to the Republic of Moldova and Ukraine. So, pretty close to the Russians who generally have blonde or light hair.