He is killed by a poison administered through a specially designed syringe, disguised as an ordinary fountain pen. The poison was so potent that it killed him instantly.
However, his death is shrouded in mystery and suspicion, as he was a known practitioner of the occult and left a series of cryptic messages that suggested he might have some supernatural abilities. This leads Sherlock Holmes and Dr. Watson on a frantic chase to uncover the truth behind his death and the sinister forces at play.
Lord Blackwood's "resurrection" is a separate event that takes place in the movie "Sherlock Holmes" (2009), not in the original stories. In the film, Blackwood's death was faked using a stuntman and a hidden compartment in his coffin. His true motives were to manipulate people from beyond the grave, using a network of followers to achieve his goals.