This doesn't work with the whole body scrollbar, only with a boxed xanga.
First off, this is the coding for your own scrollbar colors, although I assume you already know this, here it is anyway..
CODE
<!-- begin code provided by createblog.com -->
<style type="text/css">
<!--
BODY {
scrollbar-face-color: #BEC9AF;
scrollbar-shadow-color: #BEC9AF;
scrollbar-highlight-color: #BEC9AF;
scrollbar-3dlight-color: #FFFFFF;
scrollbar-darkshadow-color: #A3AC96;
scrollbar-track-color: #D6DDBE;
scrollbar-arrow-color: #595959;
-->
</style>
<!-- end code provided by createblog.com -->
Code for transparency
CODE
<!-- begin code provided by createblog.com -->
FILTER: chroma(color=#BEC9AF) allowTransparency;
<!-- end code provided by createblog.com -->
add it to the boxed xanga code like so.
CODE
<!-- begin code provided by createblog.com -->
<DIV align="center" valign="middle"
style="BORDER-LEFT: #660000 1px solid;
BORDER-RIGHT: #660000 1px solid;
BORDER-TOP: #660000 1px solid;
BORDER-BOTTOM: #660000 1px solid;
WIDTH: 850px;
HEIGHT: 470px;
BACKGROUND-COLOR: #FFFFFF"
FILTER: alpha(opacity=100);
OVERFLOW: auto;
FILTER: chroma(color=#BEC9AF) allowTransparency;>
<!-- end code provided by createblog.com -->
In the
FILTER: chroma(color=#BEC9AF) allowTransparency option, choose the color of the part of the scroll bar you want to make transparent. This color matches the scroll-bar face color and so that in turn will be transparent. Be careful though, choose colors you won't use anywhere else, it is not limited only to the scrollbar, it will turn everything of that color to transparent.