One thing this recent BLM matter movement has highlighted are different experiences of minority ethnic groups, with a focus on anti blackness in particular. This is a wide ranging issue that is played out in all areas of civic life including education, healthcare, economics and socially. Whilst every community faces racism, the data points to anti blackness being a particular issue which manifests almost everywhere.
For this, it is important that black people have their own board as the hostility they face isnt just down to their minority status. There is a specific anti-blacness which can be found in all communities, possibly in a way you wouldn't find from a South Asian community to a Chinese, or a Chinese to a Jewish for example.
A great example is a current board with a mixed raced (Black) family wanting to find a town to move to outside of London. As with most boards like this, it will have white voices saying that an area is diverse and because of that there is no racism that 'they' know of. Other times you get people talking about a large Chinese or Jewish community assuming that automatically translate to welcoming for a Black family, or even Muslim. At other times, you may get a South Asian person talking about a lack of racism from their lived perspective without understanding/respecting or being honest enough to acknowledge that the 'minority' experience isn't unilateral. Again, specifically on the issue of anti-blackness.
However, I can completely understand the want and need for all minorities to have safe spaces because of their own individual identities and experience. LGBTQ, Trans, Muslim, Jewish etc. Especially those minorities that regularly face outward hostility because of their identities.
It is not hard for Mumsnet to make a board and I'm sure almost all posters will continue to post on other boards, so I would advocate for whatever board you want to feel safe and seen, as it wont take away from Mumsnet or the majority in any way. In 2020, we should all be recognising the need to see and respect differences.
OP, what did you want to post that you felt you couldn't, or can't? Maybe you could speak to that to foster more understanding of your perspective.
Of course, all of this going on the proviso that the Black mums board isn't something you too see an issue with.