I think you mean the "blocks" menu in the admin area (admin panel, backend, you name it).
It is not related to twig files but to file
blocks is another contenttype for demonstratios purposes. You can remove them easily by deleting /commenting
this in said file:
It start about line 197 and goes to in the file distributed with Bolt 3.2
# The 'Blocks' contenttype is a so-called 'resource contenttype'. This means
# that it can be used to manage smaller pieces of content, like the 'about us'
# text, an 'our address' in the footer, or similar short blurbs of text.
# For more info, see: https://docs.bolt.cm/howto/resource-contenttype
uses: [ title ]
placeholder: 'contenttype/slug or http://example.org/'
postfix: "Use this to add a link for this Block. This could either be an 'internal' link like <tt>page/about</tt>, if you use a contenttype/slug combination. Otherwise use a proper URL, like `http://example.org`."
extensions: [ gif, jpg, png ]
Once you delete/comment said lines go back to your admin panel Configuration/check database/update database and you are done.