The Argentinian was playing on a sponsor's invite in Munich and had slipped to 837th in the world since his last victory at the 2008 Zurich Classic of New Orleans.
The Spaniard's chip was well-struck but just a few inches to the right of the hole, while Bland allowed too much break with his eagle putt to leave Romero celebrating his second European Tour win, and his first since making his breakthrough at the Deutsche Bank Players Championship of Europe in July 2007.
Detry posted a closing 66 to set the target at 16 under but when Garcia three-putted the 16th and Bland missed the green on the next, the door was open for Romero.
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Romero began his round with seven pars but birdied the eighth, ninth and 11th before a hat-trick of gains from the 13th moved him into a share of the lead.
Garcia's hopes took a hit with his second bogey on the 16th hole, while Bland had his second bogey on the 17th.
Tommy Fleetwood eagled the last to cap a confident 67 which saw him climb to 13 under alongside Nordea Masters champion Renato Paratore (68), while Henrik Stenson produced a birdie-birdie finish for a 68 which earned him a share of 10th place on 11 under par.
Romero said of his stunning success: "I'm really, really happy".
"I would have loved it to be 4 or 5 under, which I think I could have done quite easily, but, you know, it was a little bit tricky out there today", Garcia said of the windy conditions on the Eichenried course. Obviously I had a couple of little putts there at the end that looked like they were going to go in and didn't, but overall, I felt like I played nicely. "The good thing is it doesn't hurt me for the most part when I swing, which is great, but I do feel it as the day goes on from walking and kneeling down and getting up and stuff like that. I'll just try to play solid like I've been playing every day, shoot another good score and see where that puts me".
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