That said; you've just touched upon quite an interesting peculiarity of TsukiRE that we had a fair amount of arguing about back when it came out.
Idea Blood can be inherited (as per Arcueid's explanation)
Principles cannot be inherited (as per Roa's explanation)
Idea Bloods are Principles (as per Mario's explanation)
The expiation we settled for to explain this discrepancy is that Idea Blood are a "type of Principle", but they're not the same as the general kind.
For example, we know from MBTL that Arcueid has "The Principle of Focality" (極点の原理), which is the source of her powers.
Another example: DA Noel has the Principle of 望郷の鐘 (the Bells of Nostalgia) inherently, even if Arach's injection has given her some other powers (those of Rita and Paranodahlia?).
In addition, Roa has a Principle, as explained in Mario's scene (the implication that Mario drops is that his Principle is his Reincarnation, by the way!) but not Idea Blood.
So basically, we engaged with the thought that Idea Blood is some kind of "external" Principle, that can be applied to people with "normal" Principles to give them make those Principles fucking crazy and make the world break.
The thing Roa developed is most likely a technique to take someone's Principle (the generic kind) THROUGH their Idea Blood.
Normally, the succession of Idea Blood enhances the latent Principle, but in this case...
Ciel shows us the technique, hence why he says "Will I come in possession of a body strong enough to bear such dues?" after he develops it.
Ciel can defeat Ancestors, take their Idea Blood and through this, preserve their Principle. In addition, she can convert them into 大魔術 (Rituals) to use as "consumable spells". The oft comparison is that of Dark Souls boss souls...