<div class="inner-border">
    Transparent Inside Border
</div>

.inner-border {
    background: #000;
    color: #fff;
    margin: 50px;
    padding: 15px;
    position: relative;
}
.inner-border:before {
    border: 5px solid #000;
    content: "";
    position: absolute;
    top: -10px;
    bottom: 0px;
    left: 0px;
    right: 0px;
}