Deep blue colour of hypostasis is seen in death due to poisoning by:

Aniline dye poisoning causes methaemoglobinaemia which gives deep blue colour hypostasis after death. Other poisonings which cause methaemoglobinaemia are, nitrites, potassium chlorate, potassium bicarbonate, nitrobenzene, acetanilide and bromates. Ref: The Essentials of Forensic Medicine and Toxicology By K S Narayan Reddy, 27th Edition, 136, 137,139