A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. 
-Wiki

Science is not a monkey story, all things have a precise mechanism. What is the mechanism of gravity pulls light into a black hole? 

Light direction and gravity direction are the same that perpendicular to the surface of any stars, how can gravity pulls light at the same direction? If gravity can slow down the light speed, why big and small stars have the same light speed?