Cobblestones Trust chairman Graeme Gray said the blessing marked the realisation of a project that began three years ago.
"The entranceway was something we really needed to deal with, we had been going around and around in circles.
"We realised we needed more space than just an entrance."
He said the trust was "very pleased" with what they had achieved.
"It complements the museum's assets."
The ceremony included the presentation by the South Wairarapa Rotary Club of a Don Neilson painting of Cobblestones, dedicated to late member Russell Calvert, who had been the chairman of the entrance building committee at the beginning.
Rotary past president Bryan Tucker said Mr Calvert was on the design "from day one".
Cobblestones' trustee John Gilberthorpe, chairman of the exhibitions committee, said the building should be operating with all its exhibits by Labour Weekend.
"Objects in the museum will be featured here, and their stories."
Consulting on the layout are exhibition designer Craig Turvey, of 3D Creative, and project curator Ian Wards, both fresh from a Te Papa contract.
Mr Turvey said they had identified the "hero" objects, as they put it, to place in the building.
"The building itself offers a unique opportunity to showcase Cobblestones to its full advantage.
"The character of Greytown will be captured within this exhibition."
He said it was wonderful to bring their skills from Te Papa to Wairarapa.
Mr Wards said it had been a "really fun" project.
"We wanted to tell great stories to visitors, but also something locals appreciate."