when filming how do you get the string to show up well ???
Don’t have a light source facing the camera, but facing you.
Use a good color string that contrast well to your black shirt.
Should be fine.
Don’t film in a dim room. Even the brightest neon string will be dull on film in such a scenario.
If you have a blacklight, even if you have your ambient light on it will help a neon or white string to pop. In other words, you don’t have to be filming an “in the dark” video for the blacklight to help. That said, if you get a good light setup, you don’t really need it and it could be seen as gimmicky. Properly lit scene with all of the factors HaruRay mentioned and you should be fine.