diff --git a/cms/static/js/views/pages/container.js b/cms/static/js/views/pages/container.js
index 543503a598df..cec031076e62 100644
--- a/cms/static/js/views/pages/container.js
+++ b/cms/static/js/views/pages/container.js
@@ -191,7 +191,9 @@ function($, _, Backbone, gettext, BasePage,
                         xblockWrapper.addClass('is-selected');
                         break;
                     default:
-                        console.warn('Unhandled message type:', data.type);
+                        if (data.type) {
+                            console.warn('Unhandled message type:', data.type);
+                        }
                     }
                 });
             }
diff --git a/cms/static/js/views/xblock.js b/cms/static/js/views/xblock.js
index adedd2e2c093..9f63b184776e 100644
--- a/cms/static/js/views/xblock.js
+++ b/cms/static/js/views/xblock.js
@@ -58,7 +58,7 @@ function($, _, ViewUtils, BaseView, XBlock, HtmlUtils) {
                 aside;
 
             fragmentsRendered = this.renderXBlockFragment(fragment, wrapper);
-            fragmentsRendered.always(function() {
+            fragmentsRendered.done(function() {
                 xblockElement = self.$('.xblock').first();
                 try {
                     xblock = XBlock.initializeBlock(xblockElement);
@@ -180,13 +180,16 @@ function($, _, ViewUtils, BaseView, XBlock, HtmlUtils) {
             var self = this,
                 applyResource,
                 numResources,
-                deferred;
+                deferred,
+                failedJs = false;
             numResources = resources.length;
             deferred = $.Deferred();
             applyResource = function(index) {
                 var hash, resource, value, promise;
                 if (index >= numResources) {
-                    deferred.resolve();
+                    deferred.resolve({
+                        failedJs: failedJs
+                    });
                     return;
                 }
                 value = resources[index];
@@ -194,6 +197,19 @@ function($, _, ViewUtils, BaseView, XBlock, HtmlUtils) {
                 if (!window.loadedXBlockResources) {
                     window.loadedXBlockResources = [];
                 }
+                if (!window.failedXBlockResources) {
+                    window.failedXBlockResources = [];
+                }
+                // A JS resource that previously failed (e.g., blocked by an extension):
+                // mark failedJs so the specific XBlock that needs it gets skipped,
+                // but continue loading the rest of the fragment's resources so that
+                // other XBlocks in the same fragment are not affected.
+                if (_.indexOf(window.failedXBlockResources, hash) >= 0) {
+                    failedJs = true;
+                    applyResource(index + 1);
+                    return;
+                }
+
                 if (_.indexOf(window.loadedXBlockResources, hash) < 0) {
                     resource = value[1];
                     promise = self.loadResource(resource);
@@ -201,8 +217,24 @@ function($, _, ViewUtils, BaseView, XBlock, HtmlUtils) {
                     promise.done(function() {
                         applyResource(index + 1);
                     }).fail(function() {
-                        deferred.reject();
+                        console.warn(
+                            'Failed to load XBlock resource:',
+                            resource.data || resource.kind
+                        );
+
+                        if (resource.mimetype === 'application/javascript') {
+                            failedJs = true;
+
+                            // Remember this hash so subsequent fragments that reference
+                            // the same resource skip it without a network round-trip.
+                            window.failedXBlockResources.push(hash);
+                        }
+
+                        // Always continue — other resources in this fragment may belong
+                        // to unrelated XBlocks and must still be loaded.
+                        applyResource(index + 1);
                     });
+
                 } else {
                     applyResource(index + 1);
                 }
@@ -235,7 +267,20 @@ function($, _, ViewUtils, BaseView, XBlock, HtmlUtils) {
                     // xss-lint: disable=javascript-jquery-append,javascript-concat-html
                     $head.append('<script>' + data + '</script>');
                 } else if (kind === 'url') {
-                    return ViewUtils.loadJavaScript(data);
+                    // Use a raw <script> tag instead of ViewUtils.loadJavaScript because
+                    // the underlying $script (scriptjs) library invokes its callback on
+                    // both onload AND onerror, making it impossible to detect blocked
+                    // resources (e.g. from browser extensions). A raw tag fires onerror
+                    // only on genuine network failure, letting addXBlockFragmentResources
+                    // track failedJs correctly.
+                    var scriptDeferred = $.Deferred();
+                    var scriptEl = document.createElement('script');
+                    scriptEl.type = 'text/javascript';
+                    scriptEl.src = data;
+                    scriptEl.onload = function() { scriptDeferred.resolve(); };
+                    scriptEl.onerror = function() { scriptDeferred.reject(); };
+                    $head[0].appendChild(scriptEl);
+                    return scriptDeferred.promise();
                 }
             } else if (mimetype === 'text/html') {
                 if (placement === 'head') {
diff --git a/common/static/common/js/xblock/core.js b/common/static/common/js/xblock/core.js
index 1b0ab6dd84a5..f530b21f8c92 100644
--- a/common/static/common/js/xblock/core.js
+++ b/common/static/common/js/xblock/core.js
@@ -61,6 +61,13 @@
             block = (function() {
                 var initFn = window[$element.data('init')];
 
+                // initFn can be undefined when its JS was blocked (e.g. by an ad-blocker).
+                // Return null so the caller can fall back gracefully instead of crashing.
+                if (!initFn) {
+                    console.warn('XBlock init function not found:', $element.data('init'));
+                    return null;
+                }
+
                 // This create a new constructor that can then apply() the block_args
                 // to the initFn.
                 function Block() {
@@ -70,6 +77,14 @@
 
                 return new Block();
             }());
+
+            if (!block) {
+                $element.trigger('xblock-initialized');
+                $element.data('initialized', true);
+                $element.addClass('xblock-initialized xblock-initialization-failed');
+                return {element: element, name: $element.data('name'), type: $element.data('block-type')};
+            }
+
             block.runtime = runtime;
         } else {
             block = {};