At the beginning of the movie Gangs of New York, Priest Vallon tells his son not to wipe the blood of the blade after he's cut himself shaving, saying "The blood stays on the blade". I've watched the movie multiple times, but can never find what the meaning behind that phrase is. Anyone able to shed some light on this?
I just did a bit of digging on your behalf regarding this line, and there seems to be no concrete definition of it from any official source.
That said, I found several interpretations along the way, including:
1) "You cannot wash away your sins.";
2) The blood is worn as a badge of merit on the blade;
3) The blood remains on the blade and not on your hands (meaning you are relatively guilt-free).
Personally, considering the overtly religious themes within the film, I would side with option 1, especially since Priest Vallon is saying the line.
Complex Danish and/or Celtic feelings of remorse for family honor/protectiveness and achievement.