For the second year in a row, I have made jarred pickled peppers. The cloves of garlic have turned blue. Why is thus?

Harold McGee explains the reaction in detail here:

http://www.nytimes.com/2006/12/07/travel/07iht-cook.3816170.html

In "On Food and Cooking", he states blanching the garlic before pickling will minimize color development.

I find new, young, garlic, the ones that haven't been aged, are more likely to turn blue

A natural reaction from the acid. Very common and nothing to be concerned about!