view.php: We're going to refer to it as the default block template. It's the file responsible for the output of the block.

templates: This directory contains more (optional) block templates. Some blocks already come with several templates, and some only with the default block template.

A core custom template can be found in /concrete/blocks/<block-name>/templates. Custom templates are optional though, you won't find a lot of templates by default.

A custom template could also be placed in /blocks/<block-name>/templates. What's the difference to the location mentioned above? You should avoid making any modification to a file in the concrete directory. This is why it's possible to override templates by using the same path without concrete at the beginning, which will make it...