There are three things that you may need to spray for as a home gardener. They are fertilizer, fungicide, and insecticide. Here below is what and when we recommend spraying your blueberry bushes as a home gardener. We are not held liable if you use these products and your plants do not survive.
Fertilizer– There are two types of Fertilizer that we recommend you can use as a home gardener. The first product is an organic product that is a granular. It is called Espoma HT8 Holly-Tone 4-3-4, 8 Pounds and can be purchased on amazon. See instructions for how much to apply and when. There should be one application in the early spring, and then another application in the early fall. When fertilizing in the fall you should use half as much fertilizer as you did in the spring.
The other product is non-organic miracle grow product. It is called Miracle-Gro Plant Food for Azaleas, Camellias, and Rhododendrons, 1.5-Pound (Fertilizer for Acid Loving Plants) and can be bought on amazon. It should be used once the leaf buds are a 1/4 inch in size. After the first application it should be used every two weeks until the berries start to form, once the berries form you should stop fertilizing until they all the blueberries fall or are picked. Once all the blueberries are gone you can once again fertilize every two week until the middle of august(this is for Kentucky or zones similar).
Fungicide– If the leaves begin to discolor it may be a fungus issue. To spray for fungus you can use a product called Bayer Advanced 708510 Serenade Garden Ready-to-Use Disease Control, 32-Ounce which can be bought on amazon. See instructions on how to use. Use as needed.
Insecticide– Insecticides should only be used as needed. Insects will generally be there worst during the summer & fall months. The product we recommend using for home gardeners is called Garden Safe 10424X Insecticidal Soap Insect Killer Spray, Ready-to-Use, 24-Ounce, ready to use. It is a garden safe product and can be bought on amazon. If you start seeing bugs on the leaves, or little holes in the leaves then it would be time to spray the plant.