Green’s Dictionary of Slang

to make an error, practical or behavioural.

1917		Wodehouse ‘Extricating Young Gussie’ in Man with Two Left Feet 28: There’s practically nothing a Mannering-Phipps can do that doesn’t blot his escutcheon.
1929		J.B. Booth London Town 196: Provided she was wholesome, passable as to looks, and had never blotted her moral copy-book.
1935		D.L. Sayers Gaudy Night 85: Now, it was the College that had blotted its copy-book and had called her in as one calls in a specialist.
1956		‘Charles Raven’ Und. Nights 202: Two first-offenders at the Scrubs had blotted their copy-books and been sent over to join the recidivists.
1966		C. Wood Fill the Stage With Happy Hours (1967) Act III: You’ve blotted your copybook once today.
1970		B. Naughton Alfie Darling 140: I don’t want to blot my copybook in a strange place.
1997		D. Farson Never a Normal Man 140: Kee behaved impeccably [...] but I blotted my copybook.
2000		Guardian G2 10 Apr. 7: Over the years many of them have blotted their copy somehow. Some ate too much dinner, some didn’t wash up.