The win completed a historic treble for the Irish horse trainer, who also won the Champion Chase and Champion Hurdle.
The 9-1 chance Indo emerged victorious by only one and a quarter length with 2-time champion Al Boum Photo in third place.
Blackmore, who happens to be the top 2021 Cheltenham rider with six victories - was hoping to emerge the first female horse jockey to win.
Selecting A PlusTard ahead of Minella Indo was perhaps trailblazing Blackmore's only wrong choice of the racing week, but it was indeed an emotional triumph for Kennedy.
The 21-year-old jockey, who has sustained a broken leg on four occasions in his brief career said: "I can't believe it. This is what I have dreamed of as a child,"
"It could always be lot worse - broken legs and broken collarbones will heal. It's obviously unfortunate, but you have to get on with it.
"It's definitely the best day of my life. I know I'm still young, but I suppose I have been in the position where I could have been winning them for a couple of years, so to get it done is brilliant."
Happy De Bromhead, who emerged victorious at Queen Mother Champion Chase with Put the Kettle On, on Wednesday, was left to absorb what he had achieved at Cheltenham.
The 48-year-old who trains his horse at Knockeen said: "It's all come together here and it's fantastic,"
"To win a Cheltenham Gold Cup is something you dream about. I think I'm still in my hotel, it's Monday evening and I'm about wake-up and nothing's even started yet. That's where I'm at at the moment, it's brilliant."
Native River, the 2018 winner, came fourth and Frodo emerged fifth, while last year's RSA Chase victor Champ and the runner-up at 2020 Gold Cup,Santini, pulled up, and it shouldn’t come as a surprise as BoyleSports horse racing already included them in their odds.
Blackmore was handed a 2-day suspension for making use of her whip above the authorized level from turning in.
A total number of twelve runners went to post, and Bryony Frost was the early leader from the front on the 6-time Cheltenham victorFrodon.
The 2019 and 2020 winner Al Boum Photo was one of many traveling fluidly, along with his Willie Mullins-trained mate Kemboy, as the race evolved.
Blackmore, who emerged the first female rider to win the Champion Hurdle after triumphing with Honeysuckle during the week, was another going pretty well on her A PlusTard.
Frodo started to fade two fences from home and Minella Indo, placed second to Champ at Cheltenham in 2020, overtook and could not be caught even up the hill.
Cheltenham Festival-winning jockey, Katie Walsh, said on BBC Radio 5 Live that Jack Kennedy is a great rider and has been riding out his heart out. Although he had suffered horrible injuries in the past but is a fantastic Irish talent who deserved the win.
The last day of the 2021 Cheltenham Festival followed the same pattern of the last three days with Irish horses taking full control.
