The ICategorizable interface although implemented implicitly in PageData isn't implemented for BlockData as blocks can be used not only as page components for as shared properties areas. Therefore they are composed at runtime via Castle Winsor Proxies.

To gain access to a block's Category propery simple cast it to ICategorizable such as

var categoryCurrentBlock = (ICategorizable)currentBlock;

Although I would recommend doing things like this in a Controller and passing a ViewModel to the cshtml file.