The Blocs don't have explicit racial requirements and only exist in terms of 'I identify with the Rhodesian/Zimbabwean Community'. The requirements aren't de jure racial - you would just need 33% of the other Bloc in your cabinet - but in practice they are. A mixed race person would have to choose their party Bloc and be judged accordingly. In practice, most are aligned to the Zimbabwean Bloc. The whole point is you can't do anything in the country if either the Settler or Native community tells you that you can't. Mixed-race members would also vote for 'Other' parties, who don't identify with either Bloc and sit outside both of them, such as the Tolstoyists or Liberals.What's the status of mixed race folk here? Would they count for either or neither? And would this either-neither deal only be applicable for the 50-50 mixed ones or would there be something like a 7/8ths 'pure' is effectively white/black? Once race-related stuff gets written into law, issues with demarcation lines vis-a-vis the mixed race populace, however small, will become an issue.
A White member of the Zimbabwe Bloc or a Black member of the Rhodesian Bloc, however, would be almost unheard of. They would exist as oddities who would not be allowed in the Cabinet without the explicit approval of both Blocs. Again, it's a bit like Northern Ireland, but even more polarized - you'd never see a Presbyterian Sinn Fein member getting a high ranking status just as you'd never see a Catholic DUP MP. Now make everyone's difference far more visual, have the settlement be in everyone's lifetime rather than several hundred years ago, and you can see why things are a lot more on edge. It's essentially a country permanently stuck on 1970s America levels of race relations.