Fermented garlic can turn blue! And it's perfectly fine, not an indicator of a ferment gone wrong. We were pretty surprised the first time this happened, to say the least! We first saw this happen in a batch of carrot escabeche. We set up the batch to pickle just as we always have, nothing out of the ordinary or out of protocol. When we were examining the finished product, however, we were all surprised to see, among the orange carrots, red and green peppers, and purple onions, cloves of garlic ranging from a mild aqua through blue-green to out and out blue. An internet search quickly revealed that many other folks were getting similar results and that a relatively simple explanation was involved. The Chinese even make a jade-colored garlic for their Lada celebration, so there's a specific and controllable chemical reaction at work here.
Garlic contains sulfur and certain amino acids that, together, can create blue pigments. Reaction with copper (or other metals) and acid (such as that created by the lactobacilli) can facilitate this reaction and release these pigments. We were setting up an acidic environment that led to this color-producing reaction but cooking garlic with vinegar or lemon juice in a copper pan yields similar results. The way we process our garlic bruises it, so there isn't a uniform color change, rather the bruised parts undergo a deeper pigmentation release as the acid is able to penetrate deeper into the garlic. I'm starting to use this color change as an indicator of a good ferment, as it shows that our desired acid environment was achieved. And, in addition to allowing the fermentation that we desire, this acidic condition is also keeping botulinum nasties from setting up shop. Blue garlic means no botulism!
Here are a few good links: