The BLL is a method class collection, DAL is a database method collection layer. That means that whatever you see in BLL is communicating with DAL to perform database orientated tasks.
Within BLL you can alter methods so they give other objects/values to their caller.
First think of what your extending method should do, should it modify values in the database? If so, look at the DAL. If your extension method takes new arguments/parameters you should alter the BLL method for this too.
If you don't need any database specific operation look only at the BLL method.
For more information look at the following link, the 3-/multi tier programming pattern is explained here. http://en.wikipedia.org/wiki/Multitier_architecture.