The needle is inserted at the top level and then emerges next to the lower level of the stitch. The needle is brought out over the working thread, so forming a long straight stitch with a looped edge on the lower line. The needle is then inserted again further along the upper level. Its form suggests a mirror image of the letter L. This method of making a stitch is related to that used for feather stitches.
The blanket stitch is also known as the open buttonhole stitch.
See also slanting blanket stitch
Source: THOMAS, Mary (1934). Mary Thomas's Dictionary of Embroidery Stitches, London: Hodder and Stoughton, pp. 10-11.
GVE