You can repeal those laws – a future king could become catholic. A distant family member could be found in a far off country (house of Hanover for example) – but you cannot engineer a black person into the monarch because there is dynastic line of descent from a white family.

Sure, black people could marry into the line. The earliest that could have any effect is if Prince George chooses a black wife and has mixed race children – but the role of the King or Queen in a hereditary monarchy can only remain the race (or a blend of the race) with what is there already. Nationalities and religion and other such things are mutable characteristics but race is immutable.