Viagra can give you blue blindness BTW. No idea what those ol'boys in Scotland were using.
The Scottish used the Woad Plant, while the Roman Julius Caesar thought the Picts used some type of metal. Here is some interesting info from Wikipedia.
Woad (or glastum) is the common name of the flowering plant Isatis tinctoria in the family Brassicaceae. It is commonly called dyer's woad, and sometimes incorrectly listed as Isatis indigotica (a newer and invalid name for the same plant). It is occasionally known as Asp of Jerusalem. Woad is also the name of a blue dye produced from the plant. Woad is pronounced /ˈwoʊd/, to rhyme with road.
In Europe, woad was the only source for blue dye available until the end of the sixteenth century when trade routes began bringing indigo from the far east.[citation needed]
The first archaeological finds of woad seeds date to the Neolithic and have been found in the French cave of l'Audoste, Bouches du Rhone (France). Named Färberwaid (Isatis tinctoria L.) or German Indigo of the plant family (Brassicaceae), in the Iron Age settlement of the Heuneburg, Germany, impressions of the seeds have been found on pottery.
Julius Caesar tells us (in De Bello Gallico) that the Britanni used to colour their bodies blue with vitrum, a word that roughly translates to "glass". While many have assumed vitrum or vitro refers to woad, and this misconception was probably repeated for political reasons,[1] it is probable that Caesar was describing some form of copper- or iron-based pigment.[1] The northern inhabitants of Britain came to be known as Picts (Picti), which means "painted ones" in Latin, due to these accounts of them painting or tattooing their bodies.
In Viking age levels at York, a dye shop with remains of both woad and madder dating from the tenth century have been excavated.