Immediate preface of saying well, yknow race isn't "real" so everyone comes at this from their own perspective, mines gonna be pretty biased from my perspective of a metis person and how that colours my view of race as a concept, i take this as more a bug than a feature though, also this is less about voice actors and more about race in homestuck in general but so is most other things in this thread
Taking the foundation that race isn't like an inherint part of the self, this is especially true for the beta kids given well, you cant blood quantum dave he's 100% dave. so like even moreso than in real life the race of the beta kids is contextual, dave growing up at a time when over one tenth of austins population was black(overrepresented in the dense urban center particularly cheap ares, i.e where dave lives) theres yknow only logically some blackness to him, culturally speaking. johns got the whole suburban washington plus bourgeoisie background thing going on i mean, he is the one noted as actually having white 'ancestors' even if not really related to him, we get introduced as race to him as something of a novelty(I.e black president)
considering this alongside the overwhelming crushing whiteness of the internet, especially in the past, i think the white rapper line is only logical