I am so sorry for your loss! How terrible..
Can you tell me if she was acting any different in the days leading up to yesterday? Any changes in appetite, panting, etc..
Where are you located, how cold was it?
Standing by to help...
Thanks for the info...
It is hard to say if she was poisoned, and her tongue would not turn a specific color, but it could turn dark simply because there is no blood flow after passing.
I am inclined to think that your girl may have had a heart condition called cardiomyopathy. When a cat this young passes suddenly, this is the first suspicion. It is a condition that she likely had from birth, a heart defect, that progresses as they age, and goes undetected until it is usually too late. Cats are very good at hiding illness, so you would not have necessarily seen any symptoms at all. Of course there is no way to know for sure with out a necropsy, but I think this is the most likely culprit.
0 degrees celsius converts to 32 degrees farenheit here, and it is not cold enough to freeze her to death unless there was a wicked wind chill whipping.
Do not blame yourself, you had no way to know what was going on. Please accept my sincerest condolences for your loss. I hope that I have helped.