First Blood

The Hero and the Villain face off in battle. They both pull their weapons and charge at each other. After the first blow we see a shot of the floor beneath one of the combatants. A stream of blood trickles down. This is a common way to show that the battle is serious and one of the characters is injured. Often the next shot will be of the injured party touching their wound and saying "I'm bleeding?" A common trope in any show that involves a lot of bleeding.

Has jack all to do with the first Rambo installment of the same name, or the sequel, First Blood Part II.

Examples of First Blood include:

Anime and Manga

This is common in Bleach, especially Ichigo's battles.
The final fight of the final episode of Cowboy Bebop had this for both combatants.
Saito vs. Shishio, Gatotsu Zero Style!
Elfen Lied at the beginning of the Lucy vs. Nana fight.

Comics

In an old issue of Spider-Man, Peter slugs Venom in the face after being abandoned on a deserted island with his enemy. Venom actually bleeds and tells Spidey that he has "first blood".

Film

The Mark of Zorro:

Don Diego: I needed that scratch to awaken me.

Instead of finding horror in this, Bruce Lee gathers berserk strength by touching the wound licking his own blood off his fingers.
Played for laughs in Kung Pow! Enter the Fist When The Chosen One and Wimp Lo's fight is stopped, Wimp Lo declares "he is bleeding, making him the victor."
Most Errol Flynn swashbucklers had moments like this.
In The Great Race, the great sword fight had a moment like this.

Literature

In James Swallow's Warhammer 40,000 Blood Angels novel Deus Sanguinius, it is a great shock to Arkio's forces when in his duel with Rafen, Rafen draws first blood. Especially to Sachiel, who realizes that the blood is tainted, and so Arkio must be.

Live Action TV

The Firefly episode "Out Of Gas" opens with a deserted ship and Mal staggering through the hold and falling to the ground. We only find out he's wounded when we see blood dripping through the deck underneath him.

Professional Wrestling

This is the name of a gimmick match type in professional wrestling, in which the first person to bleed is declared the loser.

Tabletop Games

The Magic: The Gathering card "Sadistic Glee" has a memorable line: "First blood isn't as important as last blood..."

Video Games

In the big knife fight of Resident Evil 4, Krauser's first blow gives Leon a shallow cut on his cheek, which stays for the rest of the game.
Emma of The Last Remnant inflicts this to The Conqueror during her last stand in Athlum.

Web Original

Noka is slashed by Abel of all people, and with a sharpened stick that was ripped from a bed frame.

Real Life

In some fencing matches, the first person to draw blood is declared victor and the match ends.
- The epee was created in response to laws that duels be to first blood instead of to the death, removing the sharp tip. Instead they had three prongs to gouge out a chunk of flesh. They did still want it to be obvious when someone was struck.
- Other matches require that the blood be drawn from the torso to count as victory.