If you use this iframe code, you should add the iframe closing tag as well.

For example :

[gist type="php"]
<iframe src='https://list.ly/list/1zA4/framed?embed_type=iframe&layout=full&per_page=25&show_list_badges=false&show_list_stats=false&show_list_tools=false&show_item_sort=false&show_item_layout=false&show_item_tabs=false&show_item_search=false&show_item_numbers=false&show_item_relist=false' seamless width=700 height=2500 style='border:none' scrolling=''/> </iframe>
[/gist]

If not, the HTML page will show broken like what you facing that issue.

Perhaps you can try create another new blog post and add above iframe code what i provided and see how it goes?

By the way, if you only add the HTML block into that blog post, the system will detected that is no any blog content when you click publish blog post button, so it will return this warning message "Please enter some content for your blog entry".