[dismas]

The template that you want to edit is the navbar template. The line you're looking for is

<if condition="$show['pmstats']"><br /><phrase 1="$vbphrase[unread_x_nav_compiled]" 2="$vbphrase[total_x_nav_compiled]" 3="$session[sessionurl_q]">$vbphrase[private_messages_nav]</phrase></if>

And replace it with:

<if condition="$show['pmstats']"><br /><blink><phrase 1="$vbphrase[unread_x_nav_compiled]" 2="$vbphrase[total_x_nav_compiled]" 3="$session[sessionurl_q]">$vbphrase[private_messages_nav]</phrase></blink></if>

Although, that won't work in IE since it's not supported. Most people don't like blinking text and it makes the page harder to read for those with vision difficulties, so it's largely unused. If you still want it to blink, you could write up some javascript for it. Here is a link to w3schools.com for that.