A keyword from the X11 list
Dark violet is a CSS color keyword, written as darkviolet. Its code #9400D3 comes from the X11 color list of the Unix world. Early web browsers copied that list, and the W3C later made it part of CSS.
The Wikipedia article on shades of violet treats it as pigment violet. That is the violet you got from paints and colored pencils in the 1950s. Brighter violet pigments only became common for artists in the 1970s.
Darker than violet? Not really
The names can mislead you. In CSS, violet is a pale pinkish lilac, so darkviolet is much stronger and more saturated, not just a deeper copy of it. Two neighbors in the CSS list are dark orchid, which is a bit grayer, and blue-violet, which leans toward blue. French violet is almost the same shade.
White text works
White text on dark violet has a contrast ratio of 6.56 to 1. That passes the WCAG level AA rule for normal text. Try other text colors on it with the contrast checker.

