If the separation continues or keeps returning, it's worth having the nail examined.
Pay Attention to Dark Pigmentation That Doesn't Go Away
There is another reason persistent dark nail discoloration shouldn't automatically be dismissed as a bruise.
In rare cases, a dark streak or patch beneath a nail can be associated with subungual melanoma, a form of melanoma that develops around the nail unit.
Most dark nails are not melanoma. A previous injury, pigmentation, infection, and other conditions are much more common explanations.
Still, medical evaluation is particularly important when:
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A new dark streak appears without an obvious injury.
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The pigmentation becomes wider or changes over time.
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The color is irregular rather than uniform.
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Dark pigment appears on the skin surrounding the nail.
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The nail becomes progressively distorted or damaged.
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The discoloration remains in place instead of growing outward with the nail.
These signs don't mean you have cancer. They simply mean that the cause should be identified rather than assumed.
Your Nail Can Give You a Timeline
One useful thing about fingernails is that they grow slowly.
If a dark area was caused by an old injury, new nail typically grows from the base and gradually pushes the affected portion toward the tip.
Over time, you may therefore see the discoloration move outward.
This can help distinguish an old traumatic injury from some other causes of persistent pigmentation.
Taking a photograph every few weeks can also help you notice whether a nail is actually changing. If the appearance continues to worsen or you aren't sure what you're seeing, a dermatologist or other healthcare professional can examine it properly.
When Should You Get It Checked?
Don't panic if you notice a dark thumbnail. Instead, consider what happened before the change appeared.
If you recently injured the finger and the discoloration developed afterward, trauma may be a reasonable explanation.
But it's a good idea to seek medical advice if:
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The nail turns black without an identifiable injury.
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The discoloration keeps getting darker or larger.
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The nail repeatedly cracks or crumbles.
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The nail separates significantly from the nail bed.
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There is persistent pain or swelling.
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The skin around the nail becomes red or warm.
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Pus or other drainage appears.
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The nail develops a new unusual dark streak.
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The problem doesn't improve as the nail grows.
A healthcare professional can determine whether the problem is related to trauma, infection, inflammation, pigmentation, or another condition.
Don't Try to Fix a Black Nail Yourself
One of the biggest mistakes people make is trying to puncture or remove a damaged nail at home.
Don't drill into the nail, burn a hole through it, aggressively pull it off, or use harsh chemicals to try to remove the discoloration.