There is a very simple technique for sewing satin blanket binding with a 'sewn' in mitered corner. I did this last year on several blankets and now I cannot remember how and I cannot find the tutuorial I used. It does not leave an open space in the miter.
This uses purchased satin binding. You slip the binding over the edge of the blanket all the way to the fold and pin in place. Sew it down to the corner catching both the front and back sides with a single seam. When you get the the corner, remove from the sewing machine. The binding is then opened and folded back onto itself and sewn from the bottom edge to the outside corner at a 45 degree angle which creates the perfect sewn corner. Kripes, I hope this sounds familiar to someone....I really need to find this tutorial..... Mary