Blackwork, sometimes historically termed Spanish blackwork, is a form of embroidery generally using black silk thread on plain weave linen fabric. Some other colours were also used on occasion as well as metallic threads for accents.
It is thought to have originated in Spain and rose to its popularity in the sixteenth century in England. Although poet Geoffrey Chaucer refers to this type of embellishment in his Canterbury Tales prior to the 1500”s it is believed that it was the arrival of Catherine of Aragon to England when it's popularity blossomed.
Historic blackwork embroidery is rare to find well-preserved, as the iron based dye used was corrosive to the thread and there are currently no conservation techniques that can stop the decay. Other countries outside of England contained less iron in the black dye so blackwork worked using non-English silk tends to survive in better condition.
Today the term blackwork is used to refer to a technique rather than the colour combination. Usually a counted thread stitched on even-weave fabric is used.
Any black thread can be used, but firmly twisted threads give a better look than embroidery floss.
The stitches used for counted thread blackwork are double running or Holbein stitch, backstitch, and sometimes stem stitch.
Many books are available to help the inspired embroiderer with how to get started and a variety of patterns to follow but if you are adventurous make your own design.