I did make an assumption there. Alchemy makes sense, Victor Frankenstein was an alchemist, or at least had knowledge of alchemy.
Universal Studio's introducing the electricity angle could be an example on how legends being warped can affect the Servant. If we assume that Frankenstein's monster was created by alchemy, then Universal Studios introducing the electrical angle warped the servant. Like Hans Christian Andersen, the legend of Frankenstein's monster was warped so that instead of using alchemy, Berserker of Black had the electricity based Noble Phantasms Bridal Chest and Blasted Tree.