The article was written a while ago and may be a little bit outdated but still read it again as there are lot of useful things in it, all the basics are there and didn't change
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.
PS: i am not part of JBoss team so whatever i say may be in contradiction with JBoss "spirit"