Blueberry roots are very fibrous and really like to have good drainage. Sand would only help if drainage is an issue?
Peat moss as mentioned is a great topping.
We do have a clay base on our property but above it is approx 12" of really nice top soil.
Every year in mid to late February we add a topping of peat moss / mulch mostly to insulate the shallow roots from the hot sun.
We have 5 blueberry bushes, all different varieties. As a result we eat and freeze blueberries from late June right into mid september. I have pruned them each year so that they are a long continuous hedge. Also I keep them at a height of no more then 5-6 ft. max.
