Use four (4) plus signs for the block method
++++
<iframe class="imgur-album" width="100%" height="550" frameborder="0" src="//imgur.com/a/2JDxe/embed"></iframe>
++++
NOTE: I could not get the iframe to render.
++++
<blockquote class="imgur-embed-pub" lang="en" data-id="a/2JDxe">
DevAlpha - Gallery 1.1</blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>
++++
Or use [pass]
[pass]
<blockquote class="imgur-embed-pub" lang="en" data-id="a/2JDxe">
DevAlpha - Gallery 1.1</blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>
The triple-plus looks like it is only for inline
The markup +++<u>underline me</u>+++ renders as underlined text.
more info:
http://asciidoctor.org/docs/user-manual/#passthru