The categories widget is not working with Content Blocks. For performance reasons we have tried minimizing the amount of data being pulled in our ContentSelectors throughout Sitefintiy. Loading all the avaialble tags and categories and filtering by them woul impose slower loading of the data in general.
If you want you could pull all content blocks filtered by specific tag/category in a custom widget programatically. For example:
var manager = ContentManager.GetManager();
var myBlocks = manager.GetContent().Where(contBlock => contBlock.Status == ContentLifecycleStatus.Live && contBlock.GetValue<
is the ID of your tag to filter by. To use the above sample you'll need to add a using Telerik.Sitefinity.Model in your class, to be able to use the extension method GetValue<T>().
In a nutshell this would allow you to bind a filtered collection of shared content blocks to any suitable control and display the actual HTML content.
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