He also handed him a 15-year consecutive sentence for assaulting a youth at a party prior to the shooting.
Felony murder is a murder committed during the commission of a felony, such as robbery, burglary or kidnapping.
Washington County Deputy State's Attorney Joseph Michael said that although Blankumsee did not fire the fatal bullet, he was responsible for coordinating the armed robbery and for shooting at the group of youths and young adults that night.
"So dangerous were his actions that everyone assumed he was the killer," Michael said.
Tyshawn Jones, 23, whose last known address before his arrest was 113 S. Locust St., was convicted in January of first-degree felony murder and second-degree depraved heart murder in Dennis' death. He admitted to firing a .22-caliber gun into the air in the courtyard. Dennis was killed by a single .22-caliber bullet to the chest.
A date for Jones' sentencing has not been set, according to court records.
Blankumsee's sister, K. Blankumsee, 28, wearing a T-shirt with her brother's photograph and "Free Azaniah" written upon it, told Beachley that she didn't think it was fair that her brother was "being convicted of this crime that he didn't commit."
"They need not take my brother's life for something he didn't do," K. Blankumsee, who was teary, said. "He has never killed anyone."
She offered condolences to Dennis' family and friends.
Gena Snyder, Andrew Snyder's mother, had told Beachley that her son has been emotional since the night he was robbed and his cousin was killed.
"It should never have happened. Andrew was 13 years old and he could have been shot, too," she said.
Azaniah Blankumsee, who was slumped in the defendant's chair, gave a small nod in response.
Beachley told Gena Snyder that better parental supervision might have prevented the night's events.
Beachley told Michael, "I'm not suggesting that it mitigates Mr. Blankumsee's actions. But the court should be the conscience of the community. It's a reflection on parents and parents' lack of responsibility. There are two sides to every story... Had there been proper parental supervision, (Snyder) would have been home in Waynesboro watching a PG-13 movie."
Jessica Hose, 20, Blankumsee's fiance, who was also wearing a "Free Azaniah" T-shirt, asked Beachley to "take into consideration our daughter." She cried as the judge pronounced sentence.
Assistant Public Defender Brian Hutchison, who represents Blankumsee, said he plans to appeal.
Brooke Snyder, 20, who held the party at her apartment the night prior to the shooting, was crying in the hallway after the hearing.
"At least (Blankumsee) gets to breathe. Jonathan gets to sit in the coffin 6 feet under and doesn't get to see light," she said. "His family doesn't have to live in fear like ours does."
Joshua Dennis, 17, Jonathan Dennis' brother, said, "My mom - she cries every night. All I have left is a tattoo on my arm to remember him."