Those poison packs can be anywhere. Some cities put them around outside to kill rats. And also, mice can get in and out of houses.
It is that kind of colour. The sack is still outside but I don’t think I have the stomach to dissect it. It seems that “something it ate” is the best answer so far.
…
One of my former cats would leave almost-dead-but-still-quivering mice in my shoes. Imagine my surprise. Even though the cat’s been gone for 10 years, I still shake out my shoes before putting them on.
Mousse chocolat?
Kudos on great subject hed.
One of Old Blue’s descendants? In which case, your cat just ate a rare survivor of a species one animal short of extinction. Props!
A Blue Penguin? AKA, the Little Penguin, or Fairy Penguin, at 13" x17" the smallest Penguin in the world and certainly the cutest? In which case, congrats again for a bad-ass cat! Kitty Söze!
Pommade de beurre moisi?
To me it’s obvious: bleu cheese.
Presumably you have a Kliban cat. Were the tiny feet nibbled on?
I’ve seen what you are talking about and always assumed it was the liver.
Liver’s aren’t blue (or spherical). They’re liver-colored.
livers are also one of the hugest organs.
I understand what you are saying. I wasn’t really inspecting it to close but to my eye it looked like the thicker round portion (lobe) of the liver had a green/emerald hue after sitting out in the air, while the flat thin portions remained liver colored. You could confuse this as the round portion being the whole organ and the other stuff just being assorted flesh.
And just what color is ‘liver colored’? 
Folks - this is an Australian mouse, its bound to be a bit weird.
Anyway moving on…organs can change colour after cat er… death. My first thought was a liver, but maybe a kidney, or a testicle - they would be small.
My anatomy experience is limited to helping dad kill, hang, and dress the occasional sheep for the dinner table a few decades ago. I have this lingering memory of blue shapes which were kidneys encased in fat after the carcass cooled.
And is the head missing?
[nitpick]
Feets
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Quote without comment.
Maybe he had blue balls.