Think of a blending board as a layering or mixing board. Blending boards are very good for creating layers of fiber and color to create interesting effects in the finished yarn. Self striping yarn is very easy to spin from fiber mixed on a blending board. Colors can be laid out in stripes along the board then rolled off of the board into a rolag*. The rolag can be spun from the end and each color will come off of the prep one at a time with some blending between color changes to create a nice transition from one color to the other. Blending boards are also very good at creating chunky and textured rolags from fibers that would not normally blend well together. It is also a great way to use up left over bits and bobs of fiber that would be difficult to use otherwise.
A pseudo mixing effect can be created on a blending board by making very thin layers of different fibers and sandwiching them together. Because the fibers aren’t mixed, but only layered, a spinner has to be very careful when spinning the fibers as to prevent each layer from spinning separately, particularly if there is a pronounced difference in the lengths of the fibers used.