CCM and Style Issues1 user found helpful
your site root/concrete/blocks/ you'll find here a list of the blocks that come standard with C5.
If you want to edit these the best thing to do is to make a custom template for the block you are trying to edit.
I keep forgetting I can make a custom template and it sure is a safer way to proceed. Anything to avoid tampering with original files sounds like the way to go.