Aloha Jenine
According to Sunset Western Garden book some Hydrangea's will grow in Sunset zone H1.That is 2000 ft elevation or above. H1 is not shown on the Island of Oahu. To me that means they would be marginal at best but worth a try if they are showing up in the Oahu garden shops. It might be a new variety that has adapted.
Good Luck
Phil

Unfortuatley garden centers will sell anything they think they can make a profit on. When I lived in Kaneohe, I bought a fuchia at Star Market Nursery. It was unsuited to the climate and died slowly. They also sold lily bulbs but not one ever came up.
Now here in Northern Ca they commonly sell hibiscus and other tropical shrubs that will not last one winter in most locations. I happen to live in the warmest microclimates in my region and they usually come through the winter for me but my best friend who loves 40 miles away can not keep a hibiscus alive during the winter to save his life. He treats them as annuals, bying new 1 gallon plants each spring and potting them up to 5 gallon size by July. A lot of tropicals have long been treated as annuals in northen climates, being replaced each year. Pelorgonium, impatiens, begonia, pentas, fuchia, salvia, vinca and cuphea being the most common. I know that the concept of an annual is strange to Hawaiian gardening, but maybe these hydrangeas are intended for just one season since hydraneas need a cool period to rest and are likely to decline if they don't get it. A lot of flowering plants grown in the florist (rather than the gardening trade) are like this, indended to be tossed after they have finished blooming and are not even suited to garden culture.
Hydrangeas are one of my faves, especilally the blue ones, so enjoy them while they last.

There's lots of them in Waimea on the Big Island. They grow as hedges there. Waimea is high elevation (2,000 feet or more?) cool, windy and moist. I have several growing by the side of my house on the Hamakua coast. 400 foot elevation, half a day of sun, deep soil and lots of water. They were blue until I did some cement work near them. Then they turned pink for about a month and went back to blue. They've only been there for a year or two, so I don't know if they will survive for a long time there or not.