That's why it's important to look at the entire pattern, rather than focusing on color alone.
When did it begin?
Was there an injury?
Is it painful?
Is the nail growing normally?
Is the dark area moving toward the tip?
Is the surrounding skin changing?
Those details can be much more informative than a photograph by itself.
The Bottom Line
A black, cracked, or crumbling thumbnail can have many possible explanations, and most are not automatically dangerous.
Trauma, trapped blood, repeated irritation, fungal infections, and nail separation are among the possibilities. Rarely, persistent unexplained pigmentation can be associated with a more serious condition, which is why unusual or progressive changes shouldn't be ignored.
The best approach is simple: don't panic, but don't dismiss a persistent change either.
If your nail suddenly changes color without an obvious reason, continues to deteriorate, or develops unusual pigmentation that doesn't seem to grow out with the nail, getting it checked is the safest way to find out what's really happening.