Solveig writes:

Susanne
To embroider with black yarn on white fabric is a very old method. This embroidery technique, “svartstick” goes as far back as the 16th century and most likely England or Spain, where they used to embroider with black and white on both the clothing and furnishing fabrics.
In Sweden this technique can be found on neckcloths to the old costumes from Dalarna. Usually “svartstick” is made in geometric shapes, squares and stars. Black tassels were sewed on to the neckclothes.
To knit mittens with black designs on white was interesting and the pattern appears clearly.
Also nice that “svartstick” is such an old decoration seam. At the Nordic Museum are examples of so-called “harsklä” from the 19th century.
(translated by ratoavig)

Mitten pattern is also available via email directly from the designer.