It's because you're adding the gradient using Mozilla prefix, which is not understood by IE:
-moz-linear-gradient(#6BA4B4, #26687B);
Therefore, IE will fall back on the background-color:black;
that is also defined for the same CSS classes (.wt-rotator .thumbnails li, .wt-rotator .play-btn, .wt-rotator .prev-btn, .wt-rotator .next-btn)
Try setting this instead (as described here):
.wt-rotator .thumbnails li, .wt-rotator .play-btn, .wt-rotator .prev-btn, .wt-rotator .next-btn {
background: -moz-linear-gradient(#6BA4B4, #26687B);
background: -webkit-linear-gradient(#6BA4B4, #26687B);
background: -o-linear-gradient(#6BA4B4, #26687B);
background: -ms-linear-gradient(#6BA4B4, #26687B);
background: linear-gradient(#6BA4B4, #26687B);
filter: progid:DXImageTransform.Microsoft.gradient(GradientType=0,startColorstr='#6BA4B4', endColorstr='#26687B');
}