Short answers, pulled from the story.
When a raspberry is picked, the torus (the receptacle connecting the berry to the stem) stays on the vine, leaving the fruit with a hollow core. When a blackberry is picked, the torus comes away with the fruit, so the berry remains solid all the way through.
James Harvey Logan, an American judge and horticulturalist, developed the loganberry in 1880 in Santa Cruz, California. It was a hybrid of a blackberry and a raspberry.
Mexico is the leading producer of blackberries worldwide, with nearly its entire crop grown for export into off-season fresh markets in North America and Europe. In 2017, Mexico held 97% of the United States market share for fresh imported blackberries.
Blackberries were found in the stomach contents of the Haraldskær Woman, a naturally preserved bog body of a Danish woman dating from approximately 2,500 years ago. Forensic examination of her remains identified blackberries among the foods she had consumed.
Rubus armeniacus, the Himalayan or Armenian blackberry, is classified as a noxious weed and invasive species in many regions of the Pacific Northwest of Canada and the United States. It grows out of control in urban and suburban parks and woodlands, spreading through both vegetative reproduction and seed dispersal by wildlife.
The first thornless variety appeared in 1921, but those berries lacked flavor. Commercially successful thornless cultivars were developed from the 1990s to the early 21st century by the US Department of Agriculture, with varieties including Triple Crown, Black Diamond, Black Pearl, and Nightfall.