Blue hyacinths are a growable flower in Animal Crossing: New Horizons.
This flower can be obtained via breeding (see below), but also has a special variety that can be obtained only from mystery island visits. In the game, it looks exactly like the non-island version and does not contain 'island' in the name. However, it will breed differently than the regular version.
This item is used as a crafting ingredient in 2 DIY recipes:
You can see our full flower breeding guide here.