I have to agree with @callen there. Mythic Blood is
- Minor Magical Focus
- a power that's about what you could do with the Minor Power Virtue from RoP:M
- something that isn't Withstand Casting (TMRE, p. 36) but is close enough for government work
- Personality Flaw
- Potentially access to Heroic stuff.
It's a virtue that I want to like. But almost always find some other virtue which works better.