Dead pixel
A dead pixel stays black on every color because it no longer lights at all. It shows up most clearly against a white or bright full screen color.
Dead pixels are usually permanent. Record where they are before you contact the seller since most return policies count them.
In practice
One dead pixel on a modern high resolution screen is easy to miss on normal content and hard to unsee once you have found it. Whether it counts as a defect depends on the maker’s pixel policy. Many policies need several dead pixels before a replacement, though a dead pixel near the center can count for more.
How to check
Show full screen white in the dead pixel test and scan the screen slowly from a normal viewing distance. A dead pixel stays black. Clean the screen first, since dust looks the same. Note the location and take a photo with the white screen showing. Then check the other colors to find any stuck pixels too.