So here's a more comprehensive post about the other possibility regarding Idea Blood.

Idea Blood itself is distinct from a Principle (let's just ignore Mario), and is what defines the 27 Dead Apostle Ancestors. If a Dead Apostle is to inherit it, their rank shoots up to IX, and they take the place of the previous Ancestor. However, they do not inherit the Skill of the previous Ancestor (as per Roa), but instead only gain the power to become an Ancestor.

(Idea Blood is perhaps related to the Crimson Moon. 「月から地上に落ちた真紅の染み」<--- This. In this case, an Idea Blood is more of a literal crown than perhaps we thought earlier. It is a 'thing' that marks the person as an Ancestor, rather than the Skill of the Ancestor. Again, lets ignore Mario.)

Roa's research then, is simply a question of how to preserve that Skill together with the Idea Blood. Ciel, using Roa's knowledge, shows us the results of this, having acquired the Idea Blood of Ancestors, able to somewhat turn them into usable Magecraft.

The questions then become different. Why does Mario then say that a Principle and Idea Blood are the same thing? Doesn't Ciel having Idea Blood and using their Skills even prove the two are irrevocably connected? What exactly did Roa do to Vlov then? Perhaps just a trick for him to overcome the age problem? Is that related to his research into Succession, or not? Was Lord Rozay-en's worries simply that he couldn't have real kids? Why does Arcueid call the defeated DAA by their Principle, instead of their Idea Blood?