Subsequent to my previous response I discovered at

http://everything2.com/title/Garlic+oil

that the reaction that produces the blue colouration may produce toxins making me wonder if the blue colour is prussian blue or a derivative.

This also happened to me.

In my Freegan foragings I found a large quantity of reasonably fresh garlic.

Having no refrigerator or freezer at the time I decided to pickle the blanched and peeled cloves to preserve them for later use.

They turned a number of shades of blue, from turquoise to purple.

I attributed this to a chemical reaction between the acetic acid (vinegar) and organic compounds in the garlic behaving in a manner similar to an indicator (of acidity - like litmus).

I didn't bother to identify the compounds.

The garlic's new colour made it unappetising so I had to re-disposed of it.