what means "Key" in color picker dialog and why is this "Black" sometimes ?

l10n
#1

I have this string in libo_ui-7-3-cuimessages-ro (LO 7.3):

#. aFvbe
#: cui/uiconfig/ui/colorpickerdialog.ui:684
msgctxt "colorpickerdialog|label8"
msgid "_Key:"

Ignoring the accelerator and the colon, this is in a few languages I understand:
- in Italian is "Nero" (i.e. black)
- in French is "Noir" (i.e. black)
- in Spanish is "Negro" (i.e. black)
- in German is "Key"

I don't know the exact place for this string in order to evaluate myself the context in actual UI, but how comes that key = black ? (what is "key" here ?)

Cristi

#2

This is the color picker dialog showing up when you click on "Custom Color" in the floating widget. "Key" is the last entry for CMY*K* setting the black value. See also https://help.libreoffice.org/7.3/en-US/text/shared/optionen/01010501.html and https://en.wikipedia.org/wiki/CMYK_color_model.

#3

I've submitted a patch to add translator notes to those entries for the
future: https://gerrit.libreoffice.org/c/core/+/130650