You might want to consider the description we are given of the community of which Kira is a part, and the way that it features elements that are both pre- and post-Ruin. This helps to present us with the post-apocalyptic setting that the novel presents us with. Although the precise nature of the Ruin is never specified, it is clear that we can link it to some kind of nuclear or environmental disaster that returned civilisation to its primitive roots. Note how the following quote describes Kira's community:
Very little had survived the Ruin, but somehow the structure called the Council Edifice had remained standing and firm. It was immeasurably old. Several windows still contained patterned glass of deep reds and golds, amazing things, for knowledge of the way of making such remarkable glass had been lost. Some remaining windows, ones in which the colored glass had shattered, were now paned in a thick, ordinary glass that distorted the view through bubbles and ripples. Other windows were simply boarded over, and parts of the building's interior were darkly shadowed. Still, the Edifice was magnificent in comparison to the ordinary sheds and cottages of the village.
Note the way that this description makes clear the way that so much knowledge and technology has been lost. The old church building that is now the Council Edifice boasts forgotten knowledge through its stained glass windows, and stands in contrast to the "ordinary sheds and cottages" that the majority of the people in the village live in. This is yet another detail that the author gives us of the setting that points to the cataclysmic nature of the Ruin and the way that we are presented with some grim vision of the future.
It takes place in an unknown time period in the future.
The "blue" is a plant to make blue dye for the threads in the robe.