It's not uncommon for black Labrador puppies to have occasional white or gray hairs scattered throughout their coat. This phenomenon is usually normal and has nothing to do with stress.
Puppies, just like humans, can have changes in their coat color as they grow and mature. These changes are often due to genetics and the way their fur develops. As your Labrador puppy grows into adulthood, the distribution and color of their coat may change. It's also possible that these white or gray hairs were present from birth but only become more noticeable as the puppy's coat grows and thickens.
Stress can be a factor that affects dogs' coats, but it is more likely to manifest in other ways, such as excessive shedding or skin issues. In most cases, a few white or gray hairs in an otherwise black Labrador's coat are entirely normal and not a cause for concern.
If you notice any other concerning symptoms, such as changes in behavior, appetite, or physical health, it's always a good idea to consult with a veterinarian to rule out any potential health issues. However, the occasional appearance of white or gray hairs in a black Labrador's coat is generally nothing to worry about.