Yeah, right now the algorithm subtracts out smooth areas, leaving sharply defined areas mostly alone. I could look into handling blackboard drawings if there is enough interest. Good point about the consistency with the email, I'll have to make them the same. Thanks for the feedback and the ideas, thanks!
You know this already, but I sent them to your email as in your HN profile.
I have no idea how important blackboard is - probably less so for the business world, but plenty of students use/encounter them.
If it's easy to work out a algorithm that cleans up the blackboard but harder to work out detecting the blackboard, your idea of two email addresses (doodle/wb@, and blackboard/bb@) would probably be a good solution.