Rewriting output of existing content block I've added a content block via the administration panel, and placed and themed it - but now I realise I need to add some extra logic to output different things depending on what user is logged in.

What hook would I use to do this, and does using a hook guarantee it will overwrite whatever content has been put in via the Block admin panel?
You can try hook_block_list_alter() hook.

Act on blocks prior to rendering.

This hook allows you to add, remove or modify blocks in the block list. The block list contains the block definitions, not the rendered blocks. The blocks are rendered after the modules have had a chance to manipulate the block list.

You can also set $block-&gt;content here, which will override the content of the block and prevent hook_block_view() from running.