.block{
position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(0, 0, 0, 0.8);
  text-align: center;
  padding-top: 15%;
  opacity: 0;
  -webkit-transition: all 0.5s;
  transition: all 0.5s;
}
.block:hover{
opacity:1;
}