When you create a post in peakd you can select the community right above thee title field. All the communities that you follow will show on the list there.

If you don't follow a community, you can go to their page and then click "write a post" on the banner.

Both ways should work :)

Third option is to add community tag (hive-142159 for Black and White community) as the first tag. It will publish in that community as well.