My perspective, the legitimacy of a contract is based on, in terms of Weberian sociology, instrumental rationality under which a rational man with universal characteristics shared with others is presupposed.
The legitimacy of a political tradition which is recognized by a group of people, is based on the faith or belief in the superiority of a set of long-last, well-practiced political conducts. The essence of its superiority is its chronical nature, no matter how it matched the reality it honoured.
In real world, the above two could co-exist in a mixed way to "legitimize" a given political authority. A secular contract could be seen as dated from centuries before, in which its 'history' and endurance have empowered its legitimacy.
As to the topic of BLOOD, I guess it drives power from both traditional beliefs and another element: charisma. I've no idea about BLOOD. But there seems to be a psycho link between visual arts and human madness.
Sorry the lib's computers dont allow chinese keyin. 