Only hero characters can contact a hero chao in the hero garden to get the angel. Only dark characters can contact a dark chao in the dark garden to get a devil chao. To get a chaos chao you need to take very good care of a chao and it will turn into one, it will go back to its egg and rehatch, and you still need to take good care of it. The black markets are random.
This may seem kind of dumb, but how do you get your chao to cocoon more than once and my friend and I have a different set of eggs in the black market. Why is that?