Index: modules/block/block.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/block/block.module,v
retrieving revision 1.415
diff -u -r1.415 block.module
--- modules/block/block.module	28 Mar 2010 11:16:29 -0000	1.415
+++ modules/block/block.module	2 Apr 2010 15:56:44 -0000
@@ -704,7 +704,9 @@
       }
     }
     else {
-      $page_match = TRUE;
+      // If no paths (or php statements) were given, default to not displaying
+      // the block unless visibility is set to "All pages except those listed".
+      $page_match = $block->visibility == 0;
     }
     if (!$page_match) {
       unset($blocks[$key]);