The phrase "first blood" has several meanings, but most of which are rooted in the actions that will result in the spilling of blood when the situation turns violent.
So to "draw first blood" is the initial advantage gained by one side in any game, contest or competition.
"First blood" could also mean the time when a man kills another man for the first time in honorable combat.
Or to "draw first blood" could mean when your opponent strikes at you first, without any provocation from you. Thus, the opponent inflicts the first harm, by which he or she causes you to bleed or be harmed.
In the 1st reading, we heard that God sealed with blood the covenant between Him and His people. Moses cast the blood of bulls on the altar (which represented God) and the other half on the people.
That symbolized, in very stark terms, the sealing of the covenant in blood. And it can be said that the people of God drew first blood when they broke the covenant, and God has every right to make a demand for their blood as punishment.
In the gospel, we heard the parable in which a man planted good seed in his field but the enemy came secretly to sow darnel among the wheat and made off.
No doubt the enemy drew first blood. We might think that it refers to the evil one. But when there is sin in our hearts and when we are under the influence of the evil one, then we have become the enemy that drew first blood against God and others.
Let us ask the Lord to help us be aware of the sin in our hearts and to cast it out so that we can bear a good harvest for the Lord and keep His covenant with us.