You can use Quick Select (Tools-->Quick Select or QSELECT) with the Properties dialog to edit an attribute in many instances of a block at the same time. Start Quick Select, and under “Object Type”, select “block”. Then under Properties, choose “Name”. Pick the name of your block from the drop-down list, and click OK. (2004 & before, you're stuck typing the name in -- sorry.) Now all your blocks are selected, and you can type the desired attribute value in the Properties, under the “Attributes” section.