blue prints (aizuri-e - 藍摺絵) (genre )ai-e
"In the late 1820s a new imported blue pigment became more readily available and affordable to woodblock print publishers. This intense blue was developed in Berlin by a color manufacturer in the early 18th century, and had been sporadically imported to Japan as early as the 1780s, primarily for use by painters. Hasui The color was known as bero, a derivation from the Dutch Berlyns blaauw ('Berlin blue'); in English it is often called Prussian blue. Unlike the natural pigments previously used for print-making; this blue was strong, vibrant, and stable. While there may be examples where bero was used on woodblock prints in the 1820s, it was not widely utilized until circa 1830 when the costs decreased and the quantities increased (apparently as a result of competition between the Dutch and Chinese importers). By 1830 the production of the landscape series, 'Thirty-Six Views of Mt. Fuji' by Katsushika Hokusai (1760-1849) was underway, a landmark series which was initially advertised as aizuri-e ('all blue') series rendered in bero. At about the same time, a relatively unknown artist, Utagawa Hiroshige (1797-1858), began his landscape series, 'Fifty-three Stations of the Tokaido.' The inarguable success of these two ambitious projects essentially mark the advent of a new genre of ukiyo-e: landscape prints."
Quoted directly from information provided by Sholten Japanese Art.