Blankets can get musty because of the moisture that they absorb. This can be from sweat, rain, or even just the humidity in the air. When the moisture is allowed to sit, it allows for the growth of mold and mildew.
To prevent this, it’s important to allow blankets to dry completely after being washed and before putting them back on the bed. It’s also a good idea to rotate them out with other blankets so that they all have a chance to air out. Using a dehumidifier in your bedroom can also help keep the humidity down and help ward off any mustiness.