Why did my aluminum foil turn light blue and slightly transparent after cooking in oven?

Some kind of chemical reaction. I'm not a scientists though. Here's a tip... don't use aluminum foil in the oven... I have a new oven and the instructions say NOT to use aluminum foil in the oven as it may be destructive to the elements. Instead of foil, use parchment baking paper, that's what professional bakers use. It looks like this...

http://www.google.com/imgres?q=cooking+parchment+p...

http://www.google.com/imgres?q=cooking+parchment+p...

It has lots of uses... can be used to line the pan that you bake a meat loaf in, or line a cake pan with it. Line a cheesecake pan with it. It is used to form cones to hold your icing for decorating cakes. Can be used to line a cookie sheet pan before placing the dough in it.

Aluminum foil is made for the oven. That you would be able to preserve aluminum foil in there for hours. The aluminum foil pan you have got are the disposable ones. You employ those once or twice and throw them away. They're going to be just best! I've used them a lot of times. They may be easy b/c if you are bringing truffles to anyone- you could convey them within the aluminum container and don't care about getting the container back.

Are you sure it wasn't mylar? Mylar is a plastic base that looks like foil. I use Alcoa foil all the time and it works great. So does parchment paper.