The blueberry leaf's sole purpose is to harvest light and make sugar. Water comes from the roots. Air enters the leaves through small holes in the leaves called stomates. In blueberries the stomates are located on the bottom of the leaves. This is an important point to remember because if those stomates close to conserve water, photosynthesis and sugar production stops.
Water is the single most limiting factor in plant growth.
Take a moment to consider the structure of a blueberry leaf. It is a thick, waxy leaf. The thick, waxy cuticle reduces water loss. The leaf is thick and the edges are smooth, which means the surface area is small and evaporation is reduced. This leaf is designed to conserve water.