My cockatiel poos black, and it seems its stressed?

I have 2 cockatiels probably 4-6 months old. but ive had them for 2 weeks. they havent eaten anything and im getting worried as they seem to be stressed. its really cold outside so when i come back from school i bring them inside the house until morning i take them back out. but recently they're poo have been coming dark brown/black, im really getting worried. i feed them seed from my hands but they only eat a little bit. not only that but they wont come outside of their cage or whistle. i read that feeding fruit is better for their digestive system but what kind of fruit or vegetables? also do you believe the change from the weather from cold to warm would effect them. most of the time they are asleep together huddling and i put them a cuddle bone and they wont even TOUCH IT! i really need them to eat that and scrape their beaks because its really long. please someone help whats happening?