Depending on the blanket's stitch and pattern, lettering can be added by sewing with yarn of thinner or thicker texture. It is simplest if the blanket base is a single crochet stitch. A crocheted chain can form the letters. With longer stitches, crochet into the body of the stitch (insert hook/needle so stitch strands are equal on either side of the hook/sewing needle). Consider checking with your local yarn store for help with your project.