The rock over your blueberries is mulch, albeit an inert one. It discourages weeds and shelters earth-worms as well as holding warmth in summer. In all but the coldest climates, it may cause overheating of the soil and injury to the shallow roots of blueberry. If you want to replace it with shredded bark, pine needles, or another organic mulch, simply pick up the rocks, pull out any weeds, and spread the new mulch over the surface. Because blueberry roots are shallow and are easily damaged if you cultivate the soil's surface, avoid the temptation to dig anything into the soil. If you want to enrich the soil, spread a shallow layer of compost or composted manure over the soil, then add your new mulch.