Glastonbury; It was probably called this name because the dye plant woad was grown around there, the acidic peat was ideal for growing this plant. When used as a dye it turns green in the dye pot but when exposed to the air it oxidises and turns blue, which fits nicely in with blue/green.
To quote; Pliny records a plaintain-like plant called glastum, with which the wives and daughters of the Britons smeared themselves...