That's why I specified "usually", not all kingdoms are Catholic or even Christian but I would presume most would have similar concepts of deriving authority from somewhere to claim their blood is blue.
I'm not sure, but what you claim seems plausible enough.
Republics typically also make these kinds of claims to legitimize their leaders' claims to power. They usually invoke the wishes of the people or even a 'general will' (https://en.wikipedia.org/wiki/General_will).
