**Core Concept**
Entry-wound blackening in firearm injury is a result of the ignition of gunpowder or other propellants, leading to the formation of soot and carbon particles. This phenomenon is a critical aspect of forensic pathology in determining the origin and nature of a gunshot wound.

**Why the Correct Answer is Right**
The correct answer is related to the ignition of gunpowder, which releases a large amount of soot and carbon particles into the air. As the bullet exits the barrel, it carries these particles with it, resulting in the characteristic blackening of the entry wound. This process is facilitated by the high temperatures generated during the combustion of gunpowder, which can reach up to 3000°C. The soot and carbon particles then deposit onto the skin, causing the blackening effect.

**Why Each Wrong Option is Incorrect**
* **Option A:** Carbon monoxide poisoning is not directly related to the formation of entry-wound blackening in firearm injury. While carbon monoxide can be produced during the combustion of gunpowder, it is not the primary cause of blackening.
* **Option B:** Gunshot wound hematoma formation is a result of the trauma caused by the bullet, but it is not directly related to the blackening of the entry wound.
* **Option C:** Gunpowder residue is indeed present at the entry wound, but it is not the primary cause of blackening. The correct answer is more specific to the ignition of gunpowder and the resulting soot and carbon particles.

**Clinical Pearl / High-Yield Fact**
In forensic pathology, the presence and pattern of entry-wound blackening can provide valuable information about the type of firearm used and the distance between the muzzle and the skin at the time of discharge.

**Correct Answer: B. Gunpowder ignition.**