.btn_black {
	display: block;
    font-size: 2rem;
    text-align: center;
    font-weight: bold;
    border: 1px solid #4d4d4d;
    color: #4d4d4d;
    padding: 12px 20px;
    width: 10%;
    margin: 40px auto;
    position: relative;
    overflow: hidden;
    transition: .2s;
    z-index:10;
}
.btn_black:before{
    content: "";
    z-index: -1;
    position: absolute;
    width: 100%;
    height: 100%;
    top: 0;
    left: -100%;
    background-color: #333;
    transition: .2s;
}

.btn_black:hover {
  color: #fff;
  background: none;
  z-index:10;
}
.btn_black:hover:before {
  left: 0;
  width: 100%;
  height: 100%;
  background-color: #4d4d4d;
}