Jason was the son of Clifford and Penelope Blossom, as well as the twin brother of Cheryl Blossom. He was captain of the school water polo team and a member of the Riverdale Bulldogs. He supposedly died under mysterious circumstances when he fell from a boat into Sweetwater River on July 4th, prior to the start of the series but as it turns out he died a week later around July 11th.
Jason grew up in Riverdale, along side his twin sister, Cheryl. Prior to the start of the series, he was in a relationship with Betty Cooper's sister, Polly. According to Betty, he meant the world to her but she meant nothing to him. The relationship ended badly, Cheryl speculated that Jason was likely the reason that Polly suffered a nervous breakdown and was sent to live in a group-home.
Jason disappeared under mysterious circumstances on July 4th. That morning, he and his sister drove out to Sweetwater River together at dawn. They held hands and walked towards the water's edge. Cheryl asked her brother if he was scared, to which he gave a slight smile and shook his head. The pair rowed a boat. According to Jughead, that was the last certain thing anyone knew, as Boy Scouts stumbled upon a distraught and soaked Cheryl huddled on the rocks near the water. She claimed she dropped her glove in the water and he tried to retrieve it for her, tipping the boat over in the process, thus drowning. However, Moose and Kevin later came across his body, which washed up on shore, while at the lake together. They discovered a bullet wound in Jason's forehead, revealing that his death was not an accident.
The police, led by Sheriff Keller, opened up an investigation into Jason's death and carried out an autopsy on the body. Somehow, the results led them to Cheryl, who went with them willingly, claiming she was guilty. It emerged that Jason did not in fact die on July 4th, as previously thought, but a week later. 
When questioning Cheryl, she said that Jason wanted to leave Riverdale and never come back. She helped him come up with the plan at Sweetwater River so that their parents wouldn't go looking for him. They made all the way to Greendale in the boat and then Jason left, saying he would contact her in a month, but he never did.
While Cheryl slept in his bed, Jason appeared in her nightmare as a ghostly figure. Like a Gothic heroine, Cheryl slowly crept down the steps of her family mansion. She entered the main room, where Jason's coffin laid, only to discover that he was not in it, but the roofing of said coffin was riddled with bloody scratch marks. As she turned around, Jason reached out for her throat, Cheryl let out an ear piercing scream in response. Of course, it would soon turn out to be a dream. During his memorial, Jughead and Betty discovered from his grandmother, Rose Blossom, that he had intended to marry Polly. 
According to Polly, Jason's parents forced him to break up with her, they didn't approve of the relationship. But when Jason learned that Polly was pregnant with his child, he was extremely happy. They had planned to run away, start their family in a new and beautiful place. They were going to meet on the other side of Sweetwater River, on July 4th, but things did not go as planned. It was later revealed that Jason's car was left on the side of the highway of route 40, when Jughead and Betty went to check it out, the vehicle was there just as Polly said, inside, they discovered Jason's varsity jacket and drugs. After realizing that they had stumbled upon evidence, Betty and Jughead call the authorities, but by the time they got there, the vehicle was up in flames.
Jason appeared during a dream sequence of Jughead's, best described as some kind of idealized, make-believe version of the American dream. He and Polly sat across the table from Jughead, Betty and Veronica. Hal and Alice Cooper were there as well. They appeared to be one big happy family. 
Jason made another appearance via flashback, according to Polly, what started out as a casual conversation blossomed into an epic-forbidden romance. She was a Vixen and he was a Bulldog. They had a wonderful relationship until their family's drama got into the mix, which tore the couple apart, as Jason's parents had forced him to break up with Polly. However, the break up was short lived as Polly revealed that she was pregnant with his child. This led to a secret engagement with Nana Rose's blessing, and her heirloom ring. At Pop's shoppe they'd made plans to run away together and start a new life, but first they needed cash. With that in mind, Jason made a one time drug delivery deal for the Southside Serpents. They gave Jason an address upstate, where he was to deliver the drugs in exchange for money. However, the drugs never made it as Jason was murdered and the drugs went up in flames with his car. 
Via flashback, Jason, Cheryl, Cliff and Penelope sipped upon the very maple syrup that their family founded Riverdale on. As the heir to the Blossom family empire, Jason was set to inherit the Blossom Maple Farms, however, his untimely demise sparked a turning point, which led many to wonder who would inherit the family business in his place. 
Jason was a moderately slim young man of medium stature, around 6' 1", with pale skin, ginger colored hair and steel blue eyes. Ironically, despite his leading position on the Riverdale Bulldogs, he was not an especially muscular individual. He was often seen wearing his blue and gold varsity jacket though. According to Veronica Lodge, Jason was "crazy handsome", Cheryl agreed, saying that he was the "handsomest".
As Jason was killed prior to the start of the series, not much information about his personality it known. However, according to Cheryl, he was the best brother anyone could've asked for, and was a genuinely nice person who didn't deserve to be tortured and murdered. Jason was very protective of his loved ones, especially Cheryl. When they were younger, Cheryl insisted on having separate birthdays from Jason despite the fact that they were twins, until one year, he randomly recommended that they combine their birthdays into one. It was not until years later that Cheryl discovered the reason for this was that no one would come to her birthday. Jason protected her feelings whenever he needed to.
- "Chapter One: The River's Edge"
- "Chapter Two: A Touch of Evil"
- "Chapter Three: Body Double"
- "Chapter Four: The Last Picture Show" (mentioned)
- "Chapter Five: Heart of Darkness"
- "Chapter Six: Faster, Pussycats! Kill! Kill!"
- "Chapter Seven: In a Lonely Place"
- "Chapter Eight: The Outsiders"
- "Chapter Nine: La Grande Illusion"
- "Chapter Ten: The Lost Weekend" (mentioned)
- "Chapter Eleven: To Riverdale and Back Again" (mentioned)
- After his body is found, Jason Blossom is buried in the cemetery on the grounds of Thornhill. Prior to the discovery of his body, his parents buried an empty casket.