Whoa! Blue onions? OK, calling all you chemists...

The culprit in your color change was the garlic. It's an enzyme thing, best as anyone can figure out, sensitive to pH, and a reaction with sulphur naturally present in both items. It can also happen when exposed to a high acid environment, so pickling can also cause it to happen. Older garlic is more suceptible to the change, since the enzymes are more broken down with age. It is perfectly safe to eat, just so you know- if it happens again. In some places, blue or greenish garlic is actually considered an epicurean delight and they go to lengths to induce the color change. Red onions and red cabbage also do the same thing- just so you are forewarned. In fact, when you cook red cabbage, you manipulate when you add in fat and vinegar just for that reason. I guess people don't notice it so much with the red onion and cabbage, because they do start off kind of red-purple anyway. But with garlic, well, it is rather disconcerting.