I Googled it and didn't find a definable answer.
The custom might be as old as Zorak the Mutant, progenitor of all people with blue eyes. Zorak was the first humanoid with blue eyes, and his genes were much in demand in his day. He was able to make a nice living traveling from village to village bartering his wares for goods and 3 hots & a cot while he plied his trade in each village. But, as is often the case with people in powerful positions Zorak got a little too big for his loin cloth; once upon beginning his bi-yearly rounds, he was greeted by a village head man and his wife - and her young blue eyed child. Afterward he traveled to other villages and was able to prosper even more by having satisfied villagers sign his multiple casts, the first marketing ploy in history, as well.