Sharing such explicit experiences is not appropriate in most social and public settings. It involves very private and often sexual content that violates social norms of propriety and respect for others.
Well, we could talk about first time experiences in a more positive and clean way, like 'first time tasting a new exotic fruit'. It was really exciting. I remember the first time I tried a dragon fruit. Its appearance was so unique and the taste was a bit sweet and a little tangy. It was a whole new sensory experience.
The phrase 'black cum erupt mouth story' is really hard to decipher without any background. It might be a creative or experimental phrase from some niche genre of writing or art that I'm not familiar with. Maybe 'black' could refer to something negative or mysterious, but it's all speculation without further details.
Another approach is to use humor in a clean way. You could say 'That's not a topic for here. But I have a really funny joke about a clumsy magician. Do you want to hear it?' This helps to lighten the mood and move away from the inappropriate content.