Yeah, with the black outline it is easier for me to anti alias the character or item I am drawing. If you look closely you can see how the lightest color doesn't touch the black outline except if it is so you can make out the shape of the objekt better. It's hard to explain but looking and more pieces of this kind will defenetly help you.
the outlining technique is kinda fascinating. Studying these, for sure.
They are soo cool! But you should have put a light background because it's kind of difficult to appreciate the characters with the default dark grey 
Reminds me of cure.