Hair doesn't grow darker because you shave it. The reasons why it appears darker is because there is more surface of each individual hair that the eye sees. When hair grows, the end is tapered- so the very tip of the strand is nearly microscopic. When you shave hair, your creating a blunt end, which means that instead of tapering off you now have a very rounded end that the naked eye sees more. So it merely appears darker. Now, there is some truth to hair growing back thicker due to shaving, but that is because the action of moving a razor up and down the skin stimulates your nerves, muscles, follicles, sebaceous glands, etc (basically, gives them a work out) and that stimulates hair growth, similar to tilling a garden. However, how much it actually stimulates it is nominal. That's why waxing, laser, and threading are better options because they rip the hair out of the follicle rather than blunt it or stimulate the skin too often.
Using an eyebrow shaper to remove peach fuzz on the face is not a bad idea, as it's a form of dermaplaning. Dermaplaning is an obscure technique not widely known that derms and esti's do occasionally to rid the face of the superficial skin cells. Think of it as awesome exfoliation. I don't rec commend using a shaper for brows or upper lip, however.
