i m unshure if this is just black pearl related, all MG styles or maybe even phpbb3 any style related but i ve found this in ucp_pm_history.html
This is the original line i m talking about
<div><div class="tbl-h-l"><div class="tbl-h-r"><div class="tbl-h-c"><div class="tbl-title">{L_MESSAGE_HISTORY} - {HISTORY_TITLE}</div></div></div></div>
The above leaves a blanco replacment on HISTORY _TITLE wich makes it looks somewhat like this ..."Message History - " (somewhat like this ..because my language file is modified there!)
I'm not sure if this is the best way to solve but to at least fill the blanc with something logical i've changed this line of code to:
<div><div class="tbl-h-l"><div class="tbl-h-r"><div class="tbl-h-c"><div class="tbl-title">{L_MESSAGE_HISTORY} - {SUBJECT}</div></div></div></div>
if anyone has a better solution i ld love to know it
EDIT: my guess is that this happens in any style for phpbb3, but i could be wrong