I have no idea what that might be. It actually looks like a vein - like what you see in light colored claws on a cat. If it is not bothering him, and you can touch it without issue, at a minimum you could send a pic of it to your vet and ask what they might believe it is. What does his gums look like, especially immediately above that tooth - any redness, perhaps some gingivitis?
It looks like a groove in the tooth that has stained. If you can, try very gently rubbing it with your nail. One of my cats has similar marks and they come off with a little rubbing.
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Pet stores don't sell CET products so you'll either need to buy from the vet or online from Amazon or Chewy. The CET mini cat toothbrush works great but it's only sold at part of a kit
There is a linear, vertical line which goes down the canine tooth. This is a way that the Vets can determine the age of kitties. Or at least one determining factors. It is nothing to be concerned about.