Blocks, such as XHTML/Data Definition blocks, aren't publishable by themselves. If you've made changes to a block, such as a navigation block, and you're not seeing your changes on your live site, you'll need to publish any pages that use that block.
For blocks used on all or most every page in a site, it may be easier simply to publish the entire site.
To find out which pages use the block, you can look at its relationships by clicking More > Relationships.
If the block is assigned to a Template or Configuration, you'll need to click through to view their relationships until you find the Content Types that use the block. Relationships of Content Types are pages, so they're publishable.
The block may also have manual relationships to pages or folders assigned that you can publish directly from its Relationships screen.
For more information on publishing related content, please check out this article in our Knowledge Base: Publishing Related Content.