hook_block() is used for defining a and displaying blocks. You define the delta when you are producing the data for $op='list' this is passed back to the hook when it it is called for $op='view'.

the $delat argument will therefor be whatever you define. If you define the keys in your list array as red, green and blue that is what will be passed if the block is displayed.

Maby looking at the example will help you.