This is the scene that I couldn't call to mind. I don't believe there are others (though there could be! my command of FK trivia isn't what it once was).

So the circumstances of the blood tears phenomenon in canon are that Serena has just murdered a man by draining his blood (high above the ground, perfectly full moon, etc.). Nick cries saline tears after draining Natalie in LK, so the immediacy of taking blood from a living person isn't the whole story of the difference... though it does ably play into the once-popular construction that Nick did succeed in becoming physically human at that moment, as evidenced by his tears and Lacroix's willingness to surrender him.