[libreoffice-l10n] what means "Key" in color picker dialog and why is this "Black" sometimes ?

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

-- 
Cristian Secară
https://www.secarica.ro

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