If WordPress is stripping iframes out of your users' Custom HTML blocks, this plugin fixes that by setting iframes as allowed tags.
Please note that stripping iframes may actually be a desired behavior for security. This plugin is not in the wp.org listing because it's not entirely clear it jives with the "All code in the directory should be made as secure as possible" guideline. Use this plugin only if you're working with known users who have legitimate reasons to be posting iframes.
Clone this project into your wp-content/plugins folder, then activate the plugin within the WordPress plugins menu.