I’ve used the reflective black tape on my bike (the stuff that looks matte in the light, not the diamond engineered reflective type).
I used it on the back of the metal bracket that olds the passenger backrest. In the light, it blends perfectly with the black bracket and pad, but it lights up at night when headlights hit it. (see pic)
I also added several diagonal stripes on the sides of my hard cases.
It sticks well to metal and plastic, but I don’t think it would hold on leather.
I also thought about adding it to my helmet (also matte black), but instead went with one of those reflective bands. At first, I thought it looked a little dorky, but when I saw how bright it lit up, I was sold. Other than my headline, it’s probably the most visible part of my bike and it works at the 10-2 angles that are the most dangerous at night.