ContentBlock is a widget and not type of content. It is actually a completely unstructured content that you can put anywhere on a page. Therefore, it is not located under the Content menu in Sitefinity.
In order to use ContentBlock, please do following:
- Create a new page
- Open it in the page editor
- In the toolbar, under the "Content" section, locate the ContentBlock widget and drag it on the page (see attached screenshot).
the Telerik team