Tries from Luke Bond, Jake Diggin, Rory Seymour and a sensational Joel Grayson hat-trick secured a fantastic comeback, having trailed by 14 points at half-time.
Horsham themselves scored five tries through Aaron Linfield, Rich Tredgett, Jordan Smith, Ed Mannhardt and Jonny Goward, but were edged out in a classic at Twickenham after an unforgettable second-half from their opponents.
“It’s an unbelievable feeling, I’m so proud,” said Blank.
“We looked like we were going to struggle at half time. We told the lads that though the scoreboard wasn’t in our favour, we thought we had the legs and showed that in the second half.
“Fair play to Horsham, they are a fantastic side and we are so happy to get that win.
“I think we deserved it, it’s an 80-minute game and we played for the full 80. I haven’t cried for a long time but I’m very close to it, hugely emotional and really pleased.”
Horsham came out the stronger of the teams with scrum-half Linfield crossing after just a few minutes.
Old Northamptonians hit back quickly with a score of their own from No.8 Bond, who powered over to level the scores after 10 minutes.
Horsham then found a purple patch in the second quarter of the match, dotting down twice, first through Tredgett and then flanker Smith as they pulled into a 21-7 half-time lead.
The Midlands side were not going to lie down and roll over for their opponents though and a top quality 25 minutes transformed the tie.
“We knew what we could do, so we knew that a quick open game would suit us,” said Blank.
“A few people have said it’s one of the best finals they’ve seen so to be a part of that, it might not sink in tonight but when they watch that back, they will realise they’ve been part of a special 80- minutes, so well done to both sides.”
Diggin crept over the line to respond quickly in the second half, before two excellent Grayson finishes in the same left-hand corner suddenly put them into a 26-21 lead.
The next score was crucial and it was Old Northamptonians who got it once again, this time from full-back Seymour and they led by 12 points with just 10 minutes on the clock.
Horsham scored another try to give themselves a chance when Mannhardt went over, but Grayson then scored his third to seal the tie, before Horsham racked up a consolation score from Jonny Gowar.
“Credit to Horsham and both sets of supporters who were so loud and vociferous,” added Blank.
“We’re buzzing and looking forward to seeing what it looks like for us next season.”
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