My prescription says my SPH is -8.00, and the optometrist suggested me to choose 1.67 lenses, buy friends told me the high index lenses can block 50% light compared to ordinary lenses, so what should I do to solve this problem, I really love those kind of thin and light high index lenses!

I really suggest you should try AR-coating, it can promote the transmissivity of your high index lenses as much as 99%, thus you will have a clear sight what ever you look though the lenses from inside or outside, and AR-coatings are always very cheap, and sometimes they are free if you buy high index lenses from their glasses store!