The Buchan Scots dialect of north-east Scotland exhibits a unique phonological phenomenon: vowel harmony is blocked by intervening consonants that have no secondary articulation or other obvious characteristic that should make them opaque to harmony. In this paper, I describe the harmony and blocking pattern based on new data from speakers of the modern dialect. After establishing this as a phonological rather than phonetic effect, I propose a synchronic analysis of the pattern and a phonetic explanation for the origin of this unusual sound pattern.