The Colour of Heaven

From the author's site (see link):
The novel is set mainly in Italy, from 1295 until 1315. An abandoned child is taken in by Theresa, the wife of a glass-maker on the island of Murano in the Venetian lagoon.
The baby, Paolo, grows up short sighted but has a distinct way of looking at the world.
He befriends a painter and sets out on The Silk Route with two friends - Jacopo, a Jew, and Salek, a Muslim - all children of Abraham, in search of the perfect colour blue, the colour of heaven.
They travel in search of understanding, sight, love and meaning.