This is one of the joys of what glass can do with colour, Gary. It's the difference between reflected and refracted light. There is no blue when the light is reflected from the surface of the glass, but when light is coming through the glass, which contains colours, those colours will stop certain wavelenghts of light coming through - so what you see is what is left over from the spectrum of the rainbow - blue light gets through, (all other colours are stopped) so that is what you see.
There can be an interesting effect in some bits of Mdina, which have a frothy yellow background with teal blue strapping on top. When light comes through these pieces, the colour you see from the addition of blue and yellow is purple.
When you mix paint, you get green from blue and yellow.