All Blacks coach Ian Foster said the decision to replace a wing with a flanker was helped by the injury captain Sam Cane suffered to his back, preventing him from playing the opening game against France.
Blackadder would reinforce the loose forward stock, he said.
"We are erring on the side of coverage with the six, seven-type area particularly with Sam's back. Even though we are confident that is not long term, having Ethan in just gives us a little bit of extra protection in that space."
Cane felt better daily, and Foster said it wasn't a long-term concern.
"Whether he'll be right for Namibia [on Friday, 15 September] I couldn't tell you at the moment but the medics don't seem overly concerned by it."
While there were some tight calves and sore bodies after the France game, Foster said, the team had come through the game well.
"The injured group is progressing really strongly so Brodie [Retallick] was a great example of that. He had his 15 minutes and came through well. We are disappointed with the result but he felt pretty satisfied and felt comfortable. He is a big tick."
However, he said, blindside flanker Shannon Frizell would not be available for the Namibia game and nor would prop Tyrel Lomax.
"The cut is healing well, and we're delighted with it. But to remind people, it's more than a cut to the skin; it's actually cut through the muscle, so it's a muscle tear that must heal. He is more likely to be on a progressive programme this coming week, and maybe in the bye week [after they have played Namibia], we can start loading him up and getting him ready for the Italian game.
"Jordie [Barrett] is getting better every single day so we will make that decision as we go along. His is going to be ongoing management so it's a matter of his progress the next two days."