It depends on your perspective. Berserk is definitely dark, with its violent and tragic storylines. But some might argue there are other mangas that could be equally or even darker based on personal preferences.
One of the darkest Marvel comics is 'The Punisher MAX'. It delves deep into the gritty and violent world of Frank Castle with very intense and disturbing storylines.
Well, for a manga to be the darkest, it often features a lot of hopelessness, a cruel and unforgiving world, and characters facing constant and overwhelming hardships. Also, the art style might contribute, with heavy use of shadows and dark colors.
Hellboy has its dark moments, but it's not unanimously regarded as the absolute darkest. The perception of darkness varies from reader to reader, and other comics might push the boundaries further in that aspect.
Some really dark horror novels include 'Pet Sematary' also by Stephen King. It explores themes of death and the unnatural resurrection that brings with it a great deal of horror. 'The Shining' by King as well. The isolation and the supernatural elements in the haunted hotel are truly spine - chilling. Then there's 'House of Leaves' by Mark Z. Danielewski. Its unique format and the story of a house that seems to be a gateway to something very malevolent make it a dark and complex horror read.
One of the darkest is 'A Song of Ice and Fire' by George R.R. Martin. It features brutal wars, political intrigue, and morally ambiguous characters. Another is 'The First Law' trilogy by Joe Abercrombie. It shows a gritty and often cruel world filled with anti - heroes. 'The Kingkiller Chronicle' by Patrick Rothfuss also has some dark elements, especially in the backstories of some of the characters and the overall atmosphere of the world.