The 20-1 winner was supposed to be ridden by Paul Townend, but he switched to Max Dynamite following Ruby Walsh's injury in the preceding RSA Insurance Novices' Chase.
Walsh came from the rear of the field to deny Sam Twiston-Davies on the giant Topofthegame in the final few strides.
Gordon Elliott's Barra was another who made late headway to claim third, while William Henry, sent off the 8-1 joint-favourite along with Max Dynamite, was fourth in the hands of teenager James Bowen.
Mark Walsh, who was claiming a first Cheltenham winner, said: "You don't want to ride winner like that and Paul would have rode him before getting on Ruby's horse, but I'm just delighted to be called up by Willie to ride him.
"They went a nice gallop and everything went to plan.
"We got the splits, he winged the last, and had enough petrol in the tank."
Mullins said: "This was the plan, if we could get him right, to get him into a big handicap.
"Paul Townend switched off him and Mark was available in the weighing room and the minute I heard he was available I snapped him up.
"He travelled the whole way. Mark said the gaps opened up for him and he had a dream run coming to the last.
"He felt the horse was just very good to him. You will see him again this season."
Successful owner Luke McMahon said: "Fair play to Mark Walsh, he gave him a brilliant ride.
"We've always liked him (Bleu Berry) but he just had a few interruptions at the start of the season.
Paul Nicholls said of runner-up Topofthegame: "It's frustrating, of course it is, but he's run a great race.
"The best thing we ever did was bring him back over hurdles. He ran very well in the Lanzarote (finished fourth), won a good race at Sandown and has just got beat today.
"I think he'll have learnt again from running today and we'll put him away to go novice chasing in the autumn now.
"I hate getting beat, but at least we've got a really exciting horse for next season."
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