Bring the needle up two intersections down to the right and complete a diagonal stitch connected to the last.
Working back up the shape, complete another identical column two threads away from the previous one.
Structure of stitch
Becky Hogg, RSN Essential Stitch Guides: Blackwork (2010) , p.50
Various Authors, The Royal School of Needlework Book of Embroidery (2018) , p.147