"She was always smiling and she was a great cook," says Cassidy who is from the San Diego area. "Being around her made you feel good. It always brought joy to the whole family."
But that joy turned to tragedy when the two stopped for cold drinks at a gas station in Amarillo, Texas.
A man in a truck, Gary Carner, 58, had been eyeing them as they made their way across the parking lot. He cut them off, grabbing Cassidy by the arm, threatening her at gunpoint.
"My grandmother defended me and tried to get him away from me," Cassidy says.
Carner turned to the 63-year-old grandmother and asked if she was prepared to die. She said no. A struggle ensued, and just as Blackenburger was able to free Cassidy from his grasp, shots rang out.
"All I was thinking was, did that really just happen?" Cassidy says. "It all felt like a nightmare."
The gas station attendant called 911 as Blackenburger lay dying on the sidewalk.
Carner drove off, kidnapping another girl two blocks away. That girl was 11-years-old.
Several witnesses were able to identify Carner's car. Just hours later, authorities surrounded him and killed him.
Meanwhile, Cassidy is left without her biggest protector. But she keeps her grandmother's memory close to her heart, wearing her wedding rings on a necklace.
"It makes me feel like I have a part of her with me," Cassidy says. "I know she's watching down on me. She will always love me."
The girl later taken by Carner was not hurt. He had apparently failed to kidnap two women earlier that day.