Incidentally you might note the lack of blue pigment in this painting.
All the ultramarine used in Europe was imported from the mines at Badakshan, in what is now Afghanistan. Extracted from the semi-precious stone lapis lazuli, it was an extremely lightfast, bright deep blue. The extraction process was complicated and labour-intensive, which added to its cost (more than gold at the time). The pigment was imported via Venice, so it was used by a lot of Italian artists, who had relatively easy access to it.
Giotto probably didn't even have the option of using it.
In later rennaisance religious art you can see that the robes of the virgin were depicted in 'heavenly' blue, which would have been read as a sign of both opulence and reverence.