It would be interesting if this were true. It might be of one those unofficial things where it's not a written rule but if you follow it then you truly love Jehovah.
It could be they don't have anyone that can do coarse or kinky hair (which any color or ethnic group may have and not just black people). Anyway, I remember in the late 80's Bethel put out the call for beauticians and specifically stated that they must also know how to do black hair. Because if you can do black hair then you can also do the hair of those that have a Jewish heritage, Philipinos, Malayasian and Latino hair (who are are also known to use product to straighten their hair).
Not all black people with straight hair use chemicals to straighten their hair or have to spend forever doing their hair.
NewChapter,
Nothing is wrong with braids. Acceptance will vary by region, even among black JWs.