green calms will turn black with age. I have no idea if they will survive or not after flowering. Some survive, some don't... Save the seed either way if seed does set.

How do I save the seeds and how do they look like?

If they set you will see little seeds on the same location. I know nothing about black bamboo but I would think the seeds are small.And you can give or sell them to me if you like lol.

Here is a good article.

Here is a link that might be useful: Bamboo garden

If you do get some and have extra, I have some room here for some if you are willing to give some away.

-Steve