You do it by creating a "blanket" div which covers the entire page, set it to have a black background, semi-transparent, and z-index higher than all other page elements. Then you set the z-index of the video player to be higher than the blanket div, so it won't be covered.
something like:
#blanket {
position: absolute;
width: 100%;
height: 100%;
top: 0;
left: 0;
z-index: 100;
display: none;
opacity: 0.5; /* Will need to use a filter rule to make this work in IE */
}
#video-player {
position: relative /* z-index doesn't work unless positioned */
z-index: 200;
}
jQuery("#blanket").show();