OP, put simply if black users of MN want a dedicated topic I see no reason not to provide it.
That said some thoughts/suggestions that I hope you'll take in the spirit of helpfulness that I post in.
Firstly, remember MN is a business.
Opening a new topic costs money to moderate and you're asking this at a time where many business (including MN) have been adversely impacted by Covid.
MN make money via advertising and they make the most money from the topics with the most visitors.
We've recently seen MNHQ cleaning up low traffic topics because they are not viable to maintain.
As such I'd caution you against a Black umbrella with sub topics to begin with.
If want this area of the site to take off you need to demonstrate its popularity and the more hits you can collate on one topic the better. By starting with a long (or even short) list of sub topics you stand to dilute its impact as it organically builds an audience.
As long as users are clear in their subject line it won't be an issue to see posts of interest to other visitors to the area.
This approach also means that rather than asking users now for a list of sub topics that may/may not take off, you can monitor the main topic and see what "categories" of posts it's attracting and their volume.
From that you are in a far better position to lobby MNHQ for sub topics based on actual user behaviour and interests having proven there is a market for them that is sustainable from business perspective.
There have been numerous references to ScotNet/CraicNet in this thread and I think it's a reasonable comparison, however only in the context that neither of these topics have sub topics.
Asking for MNHQ to "replicate" the most popular site level topics (such as style and beauty) for example is unlikely to wash from a business perspective because you're asking MN to de-value those topics by splitting their user base and thus capacity for revenue generation.
It's by proving you can gain additional traffic under a Black topic for specific black S&B threads (as an example) because users would not have posted in the main topic that you can you create a reasonable case for a specific sub.
Lastly I don't think it's reasonable to ask for Black moderation.
Again from a business perspective this opens floodgates that are not business viable.
MNHQ can't fund only feminist mods for FWR or mods with teen children for the teenagers topic and so on.
If you want this to succeed and not be a short lived experiment (and I do support the concept) then I think you need to be realistic about what you ask for and be prepared for this grow organically and when it does you are in a better position to "up" what you're asking for.