It is a well recognised phenomenon that pickling raw garlic cloves in an acidic solution will produce a blue colour. When garlic is damaged, the main odourless compound, alliin is converted by the enzyme alliinase to form allicin , a thiosulphinate which gives garlic its smell and taste. The  blue colour is caused by a further reaction between the thiosulphinate compounds and naturally occurring amino acids in the garlic itself, forming a range of blue pigments. Kay Bagon Radlett HERTS