Good news, I found the Thunderbird forwarded message format and NewsBlur now handles it.
For new forwarded newsletters, NewsBlur should read the original forwarded headers, use the original sender as the newsletter feed, remove the forwarded message header block from the story body, and drop the Fwd: prefix from the title.
This applies to new messages from now on. The already forwarded test messages will stay as they are, but the next one should come in as Clarion West instead of Cassandra Granade.