I dont see why from what is written on AoN non metallic/chromatic dragons should not apply to arcane bloodline, as it is a matter regarding the focus spell it self, that is available fot both wyrmblessed and draconic
Also with the fact that imperiale dragons all have different traditions leds me to believe there is no an exact relation betwen the bloodline tradition and the original dragon's spellcasting. And even if there was, why am i allowed to be an arcane gold/silver bloodline sorcerer while gold/silver dragons are actually divene casters?
I ultimately think allowing non metallic/chromatic dragon bloodline to draconic sorc IS DM fiat, but more for the fact that allowing the use of non-core books like mwangi IS DM fiat (and in that case, why the killjoy?!)
Tell me if i said something incorrect or not precise, as i dont own the actual book