Facts in Hamlet
This well-known exclamation originated in 1422 in France, following the death of Charles VI and the ascension of Charles VII. The original expression was "The king is dead, long live the king!" ("Le roi est mort; vive le roi!") In England, to avoid civil war about the order of succession, the Royal Council declared that "[t]he throne shall never be empty; the country shall never be without a monarch." Thus, "long live the king!" became an English cry as well. In this case, it's less a sign of patriotism and more of a signal that Bernardo is a friend and not a foe.
Midnight has been long associated with ghosts, the time when magic is said to be at its most powerful and ghosts, demons, and witches are active. Because of these associations with the supernatural, midnight later became known as the "witching hour," a term first used in print by the American author Washington Irving in "The Legend of Sleepy Hollow" in 1820.
Hebenon, a poison of unknown origin. Shakespeare's scholars suggest that it could either be hemlock, the poison that killed Socrates, henbane, a kind of nightshade, ebony, sometimes spelled with an "h," or yew, a common poison extracted from the tree. Other scholars argue that Shakespeare's knowledge of botany was insufficient for him to know what poison, exactly, Claudius would've used; so he made one up.
"Unreclaimed blood" refers to somebody having too much blood in the body. Blood is one of Hippocrates’ four bodily humors (the others being black bile, yellow bile, and phlegm), and it is associated with the emotion of sanguinity, or excessive passion and lust. ‘Hot-blooded’ is a modern term with similar meaning.
Ophelia has begun throwing flowers, each of which have symbolic meanings: fennel means flattery, columbines mean cuckoldry, rue means pity, daisies mean false love, and violets mean faithfulness. From this, we can assume that Opehlia has been walking around with a bouquet of flowers, both like a woman in mourning and a bride to be.