The article was written a while ago and may be a little bit outdated but still read it again as there are lot of useful things in it, all the basics are there and didn't change
http://www.onjava.com/pub/a/onjava/2003/06/04/nukes.html

The best thing to do for you is to look at the existing modules, BB is a good one but might be too big, why not starting with the RSS module for instance ?

Copy the directory and play with it, you will see that writing Hello World in a block is not hard if you are familiarized with J2ee, but you may find this step very useful. From that point everything can be done.

Of course a documentation would be great, i would suggest that while you do the Hello World, please take some notes and provide a documentation, or write a Wiki page about it to help developers after you. That's the way Open Source Software grows.

Thanks.

PS: i am not part of JBoss team so whatever i say may be in contradiction with JBoss "spirit"