The "Blanc" (white ) day (or Blank as us anglophiles referred to it, as in how it appeared on the roster ) was an "unspoken" Standby. I never (in Virgin Express 1996-2000) found any reference to the exact "obligations" expected, I.E. when it started /finished, or how long you had available to show up for work, it was like an unwritten agreement that on these days if they called you showed.
I imagine it was in excess of the max duty hrs & for that reason was left deliberately "vague". I don't recall anyone ever falling foul of it by not responding, I imagine as a Contractor the consequences "may" have been survivable, for sure as a full time employee they would not have ben appreciated.