All sorts of crazy stories get told at Halloween time, including stories of how Halloween came to be as a holiday. Many explanations attempt to simplify the situation immensely, trying to find a single coherent source for everything that is today associated with Halloween. But the fact is Halloween is a mish-mash of a variety of different sources. It should also be remembered that Halloween as we currently understand it is largely an American holiday, yet the sources (and supposed sources) are European.
How did black cats get associated with Halloween? We can point to some likely influences, but we can't forget that sometimes we just catch onto fads without there being a particularly logical reason for doing so.
Why are so many holidays around Halloween focused on death? Partially because we put them there. Autumn is a logical time for people to think about the passing of life. After all, this is when the world largely dies. Annual plants die off, and perennials go into hibernation. This is the beginning of the really difficult time of the year for people dependent on local agriculture, which most people historically were.