HTML block uses a sanitizer to stop malicious codes, like script tags, iframe, etc, since html editor is used sitewide. Htmlpurifier is the plugin used.

Anyhow.... yea, iframe is html. I'm not sure if that was a smart assed remark or you were asking a question... but either way, on my site that code saves in a raw block. So I don't know what else to say. Look in webserver error logs and see what is blocking the save.