Maria Velten was born in 1916 in Kempen, Germany. A war widow, she raised six children, two of whom were born after World War II. Velten was known to be a caring mother and grandmother, often providing financial support to her family.

Between 1963 and 1982, Maria Velten poisoned five individuals: her father, an aunt, two husbands, and a partner. She used parathion, a potent herbicide, which she mixed into bilberry pudding. The pudding's dark color effectively masked the blue hue of the poison, allowing her to administer it without arousing suspicion. Velten's motives were primarily financial, but she often used the money to support her children and grandchildren.

Her crimes went undetected for nearly two decades until one of her daughters-in-law grew suspicious and reported her to the authorities. Velten was arrested on July 25, 1983. During her trial in Krefeld in September 1984, she confessed to the murders. She was convicted of three counts of murder and sentenced to three life terms plus an additional 15 years. Due to the advanced decomposition of some victims, conclusive evidence was only available in three cases.​

In her later years, Velten developed dementia and was released from prison for health reasons. She died on June 9, 2008, in a nursing home. Her case garnered significant media attention in Germany, leading to the production of a documentary titled "Das Blaubeer-Mariechen," which aired in 2009.​