I designed the Black-Letterhead theme. I should have checked this place sooner when my theme was added to WordPress.com a couple months ago. There's actually a simple solution to make header image clickable not just for the Blackletterhead theme, but for all themes with customizable headers, like Kubrick for example.
Unfortunately, the fix involves editing header.php which WordPress.com will not allow anyone of us to do. Maybe, we can convince these guys to make a one line change in header.php.
Just replace <div id="header"> with:
<div id="header" onclick="location.href='<?php bloginfo('url');?>';" style="cursor:pointer;">