How do block lists and resource lists interact?

You can apply both block lists and resource lists to the same classroom.  When both lists are combined, the resource lists are always given priority.  For example:

You block http://games.yahoo.com
You allow http://yahoo.com

Because of the allow link http://yahoo.com includes the http://games.yahoo.com within it's broader scope, you will be allowed to go to http://games.yahoo.com while the lists are both enabled. When you remove the resource list, the block list will behave normally.