The majority of the population of the Empire at anytime would have come from the Mediterranean region - Italians, Greeks, Semites, Egyptians, North Africans - and would have been dark haired. One exception might have been the Iberians. These were Celts - modern Spanish and Portuguese are a later, more ancestry - and related to the Gauls. They likely had hair colors ranging from black to dark blond. Rome invaded Iberia during the Second Punic war in 216 BCE, although it took about 200 years to completely subdue the region. During this time, lots of Celto-Iberians would have been taken as slaves. Later, Iberians became part of the Imperial population. The Emperors Trajan, Hadrian and Marcus Aurelius were from Hispania.
Gaul became a part of the Empire in the First Century BCE and Britain in the First Century CE. Also, beginning in the First Century, the Empire began inviting German tribes into its territory to defend it against other Germans.
I remember reading once that, when the first blond slave girls began appearing in the markets in Rome, the men were so taken by them that the Roman women began dying their hair blond in order to compete.
Juan;
Leaving aside all this stuff about hair, your written an excellent story. You've managed to pack a powerful tale into a very short story. We don't really need to make it any longer.