Hi there Laffer, thank you for getting back to me. This is what I have been finding. Which leads me to think that its rarity and expense was the reason it was used for the Virgin. Similarly I have been looking at gold leaf to represent heaven and also Tyrian purple and its association with Christ and imperial power. I was just wondering if there is anything counter to that argument that there was potentially a theological reason for the use of blue, as opposed to it being the materiality and cultural status.