General recommendations for mulching trees and shrubs call for a 2- to 3-inch-thick layer of organic mulch, such as bark chips, around the plant. Mulch piled deeper than this may lead to root rot and provide a home for mice and voles that love to feed on the bark.
Blueberries are different. Research from Virginia Polytechnic Institute and State University shows that adding a 6-inch-thick layer of sawdust, or softwood or hardwood mulch actually increases the growth rate and yields on blueberry plants. They didn't notice any increase in problems with disease or animal damage.
Researchers suggest starting with a 2-inch-thick layer of mulch the first year and adding an additional 2 inches each year until the 6-inch thickness is reached. During the first two years, increase the amount of nitrogen fertilizer applied to the blueberries to offset any decrease in nitrogen in the soil. After two years, the decomposition of the mulch provides enough nitrogen for plants.
For more information on growing blueberries, check out the Small Fruits in the Home Garden, a publication of the Virginia Cooperative Extension.