I used the apple crisp recipe from Joy of Baking and I'm quite sure I followed it to the dot (didn't add blackberries, though, only apples). But for some reason, after I took it out, let it cool a little and cut into it, I saw that the apple had turned a strange purple/black color! These were perfectly good (not rotten or bruised) apples that I bought a few days ago. Any reason that this could have happened?
EDIT: Here's a picture. I managed to salvage this one piece of cooked apple, since I had thrown out the rest of it - sorry for the crappy quality and excuse my face in the background! As you can see, half of it is tinged a strange purple/black!