Pearl cotton is thread, looks like crochet thread, and you can buy it in a skein like embroidery thread, where you have multiple threads woven together so you can use more than one thread in your blanket stitch. Yarn is kind of thick and may leave a larger hole that may stretch out with use over time, where the cotton thread is not as thick and won't make a big hole. Personally, I avoid doing a blanket stitch around my fleece blankets. I use a decorative blade in my rotary cutter - scallop, wave or pinking - and rotary around the edge of the blanket. Doesn't lose it's shape, don't have to worry about repairing broken stitches over time, and doesn't take a lot of time to do.