Apparently the hallucinogens of the blue dye would make them more fearless and therefore more fierce, at least according to my Scandinavian studies TA. An example he used was Ragnar Lordbrok's company using it before attacking various settlements in current day France.
As far as modern understanding of the berserkr myth is concerned, most historians agree that no substance was used as a combat drug/ hallucinogen before battle, save for alcohol and "natural" bravado-rising techniques (shouting, chanting, donning animal pelts, possibly prayer and special rituals, bashing of shields, displays of machismo etc ). As far as we know from the sagas, the effect was entirely psychological, not chemical.
There is also no archological proof of hallucinogens being widely used by Scandinavians. As for woad, it would not work well as a combat drug. Its narcotic/toxic influence is relatively weak, however, concentraded woad is very caustic, and would burn skin, not to mention mucus membranes when ingested/snorted or rubbed into gums.
Source:
Woad toxicity:
P. Fish, (2012) "Woad and it's mis-association with Pictish Body Art"
Berserkr, and viking combat-frenzy:
Fabing, Howard D. (1956). "On Going Berserk: A Neurochemical Inquiry"
TLDR: the battle-rage was probably purely psychological (a form of outwardly violent hysteria or with repeated use, violent outburst of a PTSD episode ) and resulted from warriors working themselves into frenzy, not combat drugs/hallucinogens.