Just a guess: If the blocks are in the library shared beween sites (default setting) then you need to enable your languages from the root page to enable the languages for the block structure. It is possible to define this setting only for the blocks as well but then you need to load the language settings page with an id query string parameter that matches the block root.
I'm trying to translate my blocks on a 7.1 episever solution. I have 2 languages where I try to translate the blocks on a page that has already been translated. When I press on the 'Show content not in -' it will show me the folders and the blocks, but the translate button is grayed out.