They occur in around 80% of Asians, and 80% to 85% of Native American infants. Approximately 90% of Polynesians and Micronesians are born with slate grey nevus, as are about 46% of children in Latin America, where they are associated with non-European descent. These spots also appear on 5–10% of babies of full Caucasian descent. African American babies have slate grey nevus at a frequencies of 90% to 96%.
This is false information, and easily found to be so. Just searching “blaschko lines uv light” gives several results saying it’s not true, and that there is never a source for that information in the articles, videos etc. that say it. It should also be obvious it’s not true just by most people’s experience, seeing people under UV light, via everything from clubs, movies, art, etc. isn’t rare at all, and you never see these lines.
Check anything you say before doing so, we are all bombarded with misinformation from malicious intent already, no need to pile onto that with misinformation from ignorance.