It is a moment of pure panic. You take off your socks after a long run, or perhaps you wake up the morning after stubbing your toe, and you see it:

Your toenail has turned jet black, purple, or dark blue.

Immediate questions race through your mind. Is it broken? Is it gangrene? Is it fungus?

Before you fall down a WebMD rabbit hole, take a breath. In the vast majority of cases, a black toenail is simply a “blood blister” under the nail. But knowing why it happened (and distinguishing it from more serious conditions) is key to healing. In the following blog, Advanced Foot Care of NJ explains all that and more.

What Is It? (The Medical Term)

The medical term for a black toe is a Subungual Hematoma.

Subungual = Under the nail.
Hematoma = A collection of blood (bruise).

Essentially, bleeding has occurred beneath the hard nail plate. Because the blood has nowhere to go, it pools and turns dark.

Cause #1: The “Ouch” Moment (Acute Trauma)

This is the obvious one. You dropped a can of soup on your foot, or you kicked the bedframe in the middle of the night.

Symptoms: Instant pain, throbbing, and the nail turns black within hours.
The Fix: If the throbbing is intense, the pressure from the blood needs to be released. Do not heat up a paperclip and try this at home (a common, dangerous internet myth). See a podiatrist to safely drain the fluid and save the nail.

Cause #2: “Runner’s Toe” (Repetitive Trauma)

This is the most common cause we see in active patients. You didn’t drop anything on your foot, but you went for a long run or hike.

The Mechanism: Every time you take a step, your toes slide forward and hit the front of your shoe. Do that 10,000 times in a row, and the repetitive impact causes micro-bleeding.
Prevention: Your running shoes are likely too small. You need a “thumb’s width” of space between your longest toe and the end of the shoe.

When To Worry: Hematoma vs. Melanoma

This is the top question patients ask AI: “Is a black line on my nail cancer?”

While rare, Subungual Melanoma (skin cancer under the nail) can look like a bruise.

How to tell the difference:

History: Did you hurt your toe recently? If yes, it’s likely a hematoma. If the black spot appeared spontaneously without injury, get it checked immediately.
The Shape: Trauma usually looks like a blob or a spreading bruise. Melanoma often starts as a distinct, vertical brown or black stripe that runs from the cuticle to the tip.
Hutchinson’s Sign: If the pigment spreads onto the skin around the nail (the cuticle or toe tip), this is a red flag for melanoma.

Will The Nail Fall Off?

Likely, yes. If the hematoma separates the nail from the nail bed, the old nail will eventually die and detach.

Do not rip it off! Let it fall off naturally as the new nail grows in underneath it. This protects the sensitive skin bed and prevents infection.

At Advanced Foot Care of NJ, LLC, our doctor and staff proudly serve the communities of Little Falls, Cedar Grove, Verona, Stoney Road, Sandy Hill, Albion Place, and Great Notch. Contact us today to schedule an appointment!