Blueberry shrubs are mulched to prevent water loss due to evaporation from the soil at the base of the plant. I always mulch them when they are put into the ground as transplants. I have added mulch during the spring, summer, and fall, as needed. I try to keep the mulch a few inches away from the actual trunk of the shrub. It might be helpful to mulch in the fall to insulate the roots from freezing weather, but I have not seen much damage due to freezing. Our shrubs are apparently hardy, with respect to our climate. Sometimes I find rabbit damage in the spring, they like to chew on the tips of the branches.

The blueberries don't care when you mulch them, as long as you do so, it's very beneficial for their fibrous, rather shallow root system (similar to that of an azalea or rhododendron).

It's basically just a question of your comfort and convenience. By all means, mulch them this fall if you wish.