The colour Gosu Blue is an on glaze enamel found on a type of Satsuma which used to be called Imperial Satsuma in mid to late 19 century. Gosu Blue is distinctive and very thick, that can be either dark blue, green or black depending on the firing. "Satsuma Gosu Blue" was produced in very limited quantity in Kyoto in the mid-19th century, and is now the most sought after of the Satsuma wares.
Gosu blue is usually dense, and even when thinly applied one cannot see through it, and it does not get less dense when thin.
Sandra's Treasury of Satsuma labels them as mostly late Edo (say 1850-70). Some prefer a later date (say mid to late Meiji, the hey-day of Japanese export ceramics, at least by quantity).