I'm not sure specifically which 'old black cock story' you are referring to. It could be a very specific local or personal story. Without more context, it's hard to say.
There's not enough context to really define it precisely. It might be a title or description within a very niche cultural or family - related context that is not widely known.
It could potentially come from a region where black chickens were a significant part of the local livestock. Maybe in some parts of the Caribbean or certain African regions where they have a rich oral tradition. But this is just a hypothesis. There's also a chance it's a completely fictional story made up by someone and then passed around in a small community without a specific geographical origin.
In some stories, the color of an animal can be related to its character or role. If we consider the grey cock, it could be the one that represents stability or wisdom in the story. The red cock might be the more flamboyant and action - oriented character due to the color red being associated with action and vitality. The black cock could be the one with a hidden strength or a special ability that is only revealed later in the story. But again, this is all speculation without the actual story.
I'm not entirely sure what a specific 'black cock hypno story' is. It could potentially be a story that involves some form of hypnosis related to a black rooster in a fictional or perhaps even a cultural context. However, without more information, it's hard to define precisely.