In the threadbit template, it's defined in the {vb:raw thread.prefix_rich} variable, I'm not sure if you can use that in the block though. You can always give it a shot.
Here is the complete piece of code in the threadbit template:
PHP Code:
<vb:if condition="$thread['movedprefix'] OR $thread['typeprefix'] OR $thread['moderatedprefix'] OR $thread['prefix_rich']">
<span id="thread_prefix_{vb:raw thread.realthreadid}" class="prefix understate">
{vb:raw thread.movedprefix}
{vb:raw thread.typeprefix}
{vb:raw thread.moderatedprefix}
{vb:raw thread.prefix_rich}
</span>
</vb:if>