The course is new to the Motatapu event and is normally a three to four-day tramping track.
It followed an extremely challenging route over the Motatapu Track, from Wanaka to Arrowtown and included four major climbs totalling 2790 vertical metres.
Blackhurst (43) and Dagg (42) posted the winning time of 7hr 25min 6sec.
Blackhurst said the men's strength came from taking it easy on the descents, which saved their energy and protected their muscles for the climbs.
Also new to Motatapu this year was the 15km Miners Trail that took a challenging off-road loop from Arrowtown into the high country property of Glencoe Station.
The stand-out performance of the day among the 390 competitors came from Louisa Andrew, of Dunedin, whose 1hr 30min 46sec time was good enough to bring her home second overall behind Graham Harris (1hr 25min 28sec).
The other traditional Motatapu races included a 2400-strong field of mountain bikers and 760 off-road marathoners.
Four-time mountain-bike winner Marcus Roy, of Invercargill, finally prevailed over 15-year-old sensation Anton Cooper, of Christchurch.
"Anton was pushing the pace from the start. He had me working for it. He was stronger than me on the hills but I finally got ahead 1km from the end on the last of the river crossings. I think my longer legs were helpful," he said.
Roy's winning time was 1hr 57min 55sec, just eight seconds shy of his 2008 race record.
Cooper has been identified by BikeNZ as an outstanding talent.
He won the under-17 age group at the national championships in Wellington last month and the Australian under-17 national cross-country title.
Kate Sydey, of Christchurch, won the women's mountain bike in 2hr 38min 52sec.
In the off-road marathon run, Auckland's Gus Bell returned from a two-year injury lay-off to a winning time of 3hr 04min 03sec.
He has past experience as a New Zealand mountain running representative but it was his debut at the marathon distance.
Alexandra Williams, of Christchurch, claimed the women's title in 3hr 25min 31sec.
Kathrine Switzer, of Wellington, came in at 5hr 38min 21sec.
Switzer (63) was the first woman to officially run the Boston Marathon in 1967 and is credited with championing the equality of women in long-distance running.
The Motatapu marathon was her first in 34 years.
"I'm chuffed. It was fantastic. What an experience. It was much more difficult than I was anticipating or anyone had told me or warned me," she said.
She had trained hard for the event but it didn't prepare her for how difficult the climbs would be.
"I have a level of fitness that I can run 12km every day no problem. I did a four-hour run and a five-hour run but it was a lot harder than that," she said.
Switzer will run a marathon in Greece in October, to commemorate the 2500th anniversary of the Battle of Marathon.
