token_filter module: [node:nid] for custom block?
[current-page:url:unaliased:args:value:1]

gives you the [node:nid] even when called from a block.

As per:

https://www.drupal.org/node/1265010

..."[current-page:url:unaliased:args:value:0] (The first value, of the array of arguments, of the unaliased URL of the current page)."