The Henry de Bromhead-trained runner is now the 4/1 favourite for the 2022 Cheltenham Gold Cup with Planet Sport Bet.
A close second to stablemate Minella Indo in the Cheltenham Gold Cup in March, the Henry de Bromhead-trained runner was the 11/10 favourite to make a victorious reappearance on Merseyside.
Ridden with supreme confidence by Blackmore, A Plus Tard settled well off the early pace set by Bristol De Mai and Royale Pagaille and was still cantering in behind as the race really began with four to jump.
Bristol De Mai, a three-times race winner, was the first to really give way, with A Plus Tard making effortless ground to come upsides Royale Pagaille and assume control with the minimum of fuss.
Blackmore barely had to move as A Plus Tard kicked on approaching the second-last, clearing the final obstacle in style to coast home by 22 lengths from Royale Pagaille.
Chatham Street Lad and Clondaw Castle were the only other finishers, with Imperial Aura falling at the 13the fence and Bristol De Mai and Waiting Patiently both pulled up.
De Bromhead said of the Cheveley Park Stud-owned winner: "My heart was in my mouth as normal, but Rachael was brilliant and obviously A Plus Tard was brilliant as well.
"With his preference for left-handed tracks, this just seemed the obvious place to go - I was sick of getting beat in the Fortria Chase at Navan for the last couple of years over two miles!
"This is a fantastic race, it's brilliant to win it and it's brilliant for the Thompsons (of Cheveley Park)."
A Plus Tard created his own piece of history in victory, becoming the first Irish-trained winner of the race since its inception in 2005.
De Bromhead added: "After the 10th person told me no Irish horse had ever won it before, I started to wonder why we were here at all, but he's got so much class.
"He's won a Grade One over two (miles) and a couple of Grade Ones over three. He's still only seven, so fingers crossed he's got a long career ahead of him.
"He's achieved a hell of a lot for a horse of his age. He'd been working really well and everyone was very happy with him.
"There's a great crowd here and we always get a great reception."
A Plus Tard is now set to tread a familiar route to the Cheltenham Festival, with Leopardstown the next port of call.
He snatched a dramatic victory in the Savills Chase last Christmas and a title defence is uppermost in the mind of De Bromhead, who has the King George at Kempton as a likely destination for Minella Indo.
He added: "Because of his preference for left-handed tracks, the Savills Chase at Leopardstown seems to be the sensible way to go."
Blackmore had not previously ridden around Haydock, but she enjoyed her Merseyside debut.
She said: "That was incredible - I'll definitely like coming back to Haydock from now on!
"It was a fantastic performance. He's such a classy horse and couldn't have done it any better.
"He jumped fantastically and felt superb and I was happy everywhere. It's not often races are that straightforward, but it's very enjoyable when they are.
"You only have to look back at what he's done. It wasn't his day in March, but he's proved today he's extremely good."
Blackmore admits A Plus Tard is a personal favourite, having given her a breakthrough Festival win in the 2019 Close Brothers Novices' Handicap Chase.
"He's very special to me as he gave me my first ever Cheltenham winner," she added.
"He's got gears, he stays and jumps really well. He cruises along without doing too much.
"He's already established and is improving the whole time. He did nothing wrong in Cheltenham in March - everything went right for him and Minella Indo was a better horse on the day.
"Henry has got a massive yard of extremely talented horses and he is bang up there."