I expect they could see blue as in the sky but were unable to make it as a dye in clothes say ....

I expect blue was made later when we started using chemicals to dry rather than animals or plant life .

Around 1800 green was popular on walls until they discovered that the arsenic in the colouring was killing people .They say that is what killed Napoleon ...but who knows for sure ?

In the renaissance they had blue paints for their pictures .

Think I will go and look it up.