The notion of a blue berserk mixes together two mostly-unrelated cases: that of Berserkers, or berserks, “Norse warriors who are primarily reported in the Old Norse literature to have fought in a nearly uncontrollable, trance-like fury”  and Celtic warriors, which “Julius Caesar in his Commentaries on the Gallic Wars” described as dyeing their skins blue .
Note that while woad  (or, Isatis tinctoria ) produces a nice blue dye, it is not at all clear that woad actually was used by Celtic warriors for skin dyeing. Source  says: “experimentation suggests that woad is not very well suited as a skin dye nor as tattoo ink”, and source  suggests that the vitrum coloring agent mentioned by Julius Caesar may instead have been “some form of copper- or iron-based pigment”.
 Wikipedia, Berserker
 Wikipedia, Celtic warfare
 Wiktionary, woad
 Wikipedia, Isatis tinctoria