This happens when your skin is sort of translucent and the hair root can be seen through the skin. The only thing that has worked for me is waxing. They do very small strips because my hair grows in a zillion different directions and I have normal looking armpits for about a month. It's gotten better as I've gotten older (35 now), but when I was younger it was so embarrassing and it always looked like I didn't shave. I'm blonde, but my underarm hair is dark so it was soooo attractive lol.