The Limbourg brothers (who likely died of plague) were famous for their inventive use of bold colors, including the blue featured in this folio, which was actually made from dust of lapis lazuli in gum arabic.
“Him the Almighty Power
Hurled headlong flaming from th’ ethereal sky
With hideous ruin and combustion down
To bottomless perdition, there to dwell
In adamantine chains and penal fire,
Who durst defy th’ Omnipotent to arms.”
– John Milton, Paradise Lost