CSS3

.bx-controls-direction a {
    opacity: 0;
    transition: opacity .25s ease-in-out;
    -moz-transition: opacity .25s ease-in-out;
    -webkit-transition: opacity .25s ease-in-out;
}

.bx-wrapper:hover .bx-controls-direction a {
    opacity: 1;
}

CSS2

.bx-controls-direction a {
    display: none;
}

.bx-wrapper:hover .bx-controls-direction a {
    display: block;
}