blob, v.² — Green’s Dictionary of Slang

blob v.²

[? a blob of ink that mars an otherwise faultless piece of handwriting]

(US) to make a mistake; thus as n., a mistake.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 23: blob, n. A mistake. Blob, v. t. To make a mistake.
1960, 1975		Wentworth & Flexner DAS.