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>