Give this CSS a try:
.wp-caption .wp-caption-text {
padding: 8px 10px;
margin: 0;
background: rgba(0, 0, 0, 0.5);
color:#FFF;
position:absolute;
bottom: 0;
left: 0;
}
.wp-caption .wp-caption-text a,
.wp-caption .wp-caption-text a:visited,
.wp-caption .wp-caption-text a:hover {
color:#FFF;
text-decoration: underline;
}