It's hard to say precisely without knowing more about the 'black cock story'. Maybe the content of the story goes against some social norms or her own self - image that she has in public, which makes her embarrassed.
She could be a little girl who is fascinated by animals and stories. The black cock might be one of her favorite characters in the stories she hears, and she is always eager to hear more about it.
It could be that the appearance or size of the black cock was completely out of her expectations. Maybe it was a rare breed or had some unique features that shocked her.
I'm not sure if there is a specific 'big cock black panther she fanfiction' as it's not a very common or mainstream topic. Fanfiction can cover a wide variety of themes, but this one seems rather unusual. You might have to search through various fanfiction platforms very thoroughly.
She can start by limiting her exposure to such stories. Avoid the sources where she usually comes across them. For example, if it's a certain type of book or website, stop visiting them.
The context could be a rural or farming - related one. If she lives on a farm or has an interest in farming, stories about different types of livestock, including black roosters, would be appealing. So she was going back for more of that story.
The phrase 'black cock' could potentially refer to a black rooster in a story where perhaps the sight or actions of this rooster have an interesting or captivating effect on 'her', making her react strongly, like being so engaged or charmed that it's described as'melting'. But without more context, it's hard to be certain.