There are two important changes made to how Markdown Extra works on WordPress.com, affecting code blocks:
Code blocks can use three or more back ticks (```), as well as tildes (~~~).
Syntax highlighting can be added to code blocks by specifying the code language in the first line of back ticks:
```css
.ninja {
visibility: hidden;
}
```
The list of supported languages can be found in the Posting Source Code support page.