In the study scene of Goethe’s Faust I, a young student asks for advice. Mephistopheles tells him to take notes as if the Holy Spirit were dictating. The student answers: Denn, was man schwarz auf weiß besitzt, / Kann man getrost nach Hause tragen. For what one has in black and white, one can carry home with confidence. Schwarz auf weiß, black on white, is the everyday German phrase for something in writing.
In the play the line is usually read as ironic. Mephistopheles is teasing a student who trusts paper.