It's quite surprising to me that there's only one beach with black sand on Hawaii (maybe not exactly one beach, but it's only a very small fraction). The concern is, if the whole Hawaii is volcanic, therefore made of frozen lava, the sand should be black everywhere.
I suppose the true reason is that the sand on the beaches is not "made of Hawaii" or "made in Hawaii". However, I wonder how did the white sand make it there?
Beaches in Hawaii may be made of 'black sand' derived from the erosion of volcanic rocks, of 'white sand' made by marine organisms, or a mixture of both. On the windward side of the Big Island, for example, black sand beaches are very common. The beach at South Point is almost entirely green sand composed of olivine, a common mineral found in the volcanic rocks here in Hawaii. On the Kona coast of the Big Island and on the other islands, the beaches have a range of mixed compositions, some with a high volcanic (detrital) component, some dominated by calcareous (reef-derived) sediment. Beach and submarine sands in Kailua Bay on windward Oahu (my research area) are almost entirely composed of calcarous, reef-derived material. On average, only about 5% of the sand grains are volcanic minerals or rock fragments. Every beach is unique and has its own source and type of sediments.
They even address some of the questions regarding "foreign import" here.