Blood is of course referenced famously at the beginning of the play in the opening speech from the Chorus, when he refers to the inevitable tragedy that will befall the two feuding families. Blood throughout the play is shown to refer to the violence that occurs as a natural result of the hate that exists between the Montagues and Capulets, and this is clearly referenced in this opening speech:
Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
Note here how the repetition of "civil" signposts the way that "ancient grudge" results in transforming even the most civil and respectable of families into humans who will commit acts of violence and hate. It is this rancour between the two families that is directly linked to the shedding of blood, which of course leads to the tragedy that befalls Romeo and Juliet as they become divided by insurmountable barriers. The watchman's comment in Act V refers to one example of the shedding of blood, but the play includes many others, including the deaths of Mercutio, Tybalt, Paris, and of course of Romeo and Juliet themselves. At each stage blood is the natural and logical consequence of the unreasoning enmity that exists between the two households.