She told police her rationale was: "You have ended my life, I'm taking yours".
The mother hid two firearms in her robe pockets before facing him in his bedroom, police said in a statement.
After gunning down her son, Blessing turned the weapon on his girlfriend who tried to stop the slaying, according to police.
MCSO says they found Blessing sitting in a recliner chair when they entered the home.
She is said to have told deputies that he wanted to send her away because she "had become hard to live with" since moving in with him six months previously. During the argument, she took one out and fired at her son. The two fought again before the girlfriend again knocked the pistol out of her hand.
The son's girlfriend told authorities Blessing had been living with them for six months before the incident.
The court heard that Blessing had been anxious about her son's plans to put her in a home because she 'had become hard to live with'. The girlfriend called 911. Court documents state that deputies discovered the son's body on the floor with gunshot wounds to the neck and jaw.
"This is definitely an odd one", Sgt. Bryant Vanegas told Arizona's Family.
"It is always concerning when domestic issues escalate to violence or tragic outcomes", Sheriff Paul Penzone said.
Blessing is being held in jail on a $500,000 cash-only bond and under Arizona law, the 92-year-old can be sentenced to death or life in prison without parole if she is convicted. "They are often isolated and neither predictable nor preventable".
Anna Blessing has been charged with first-degree murder.