Can only share my story: (4 years ago skin-melanoma-female Caucasian – then 57 years old)
8 monts ago: Had a 3 mm black line in my toenail, beginning from 1/3 from the nail bed. It appeared like a train: stopped and started again, so it was not a continuous line. Went to my derm (specialized in melanoma). She took a picture and asked me to come back in 4 weeks. She told me it was not looking like a melanoma.
4 weeks later: she could see it was growing out, but was not sure if it was blood coming from a laesion- maybe melanoma. So another picture was taken, followed by another check-up after 2 months, By that time it was clear that it was growing out, and days later I could take a black part out out with tweezers. Took a last picture: case closed. Now the nail is back to normal.
My advice: find a specialist in melanoma, and take pictures yourself. The fact that it appears on more nails looks like a sign its not a melanoma, but you should be 100% certain.
Marti