Markdown is a very small plugin for the Blocks CMS that adds a markdown filter into the templating engine.
Clone / download the code and place the markdown folder inside blocks/plugins. Install through the Blocks CP.
Create a plain text block to store the content. Ensure it supports line breaks. Then, in your template, pipe the block into the markdown filter:
{% for entry in blx.entries.find({ section: 'Blog' }) %}
{{ entry.content | markdown }}
{% endfor %}