Blackhat SEO, or spam-seo, is a malicious technique used to manipulate the search engine results in order to benefit a website in terms of relevance.
The payload is HTML based, thus intended for client-side use and the payload is executed after the infected page is loaded directly in the browser.
Iframes are inline frames used to embed code from another document, local or remote. Just as all other HTML markers it can be use for malicious purposes.
Malicious iframes are usually hidden by changing their position off-screen or setting its style to hidden. They most commom malicious code in those iframes are spam content or drive-by malware.
Affecting
Any type of site.
Cleanup
Investigate your site's files in order to find any hidden iframe that wasn't added by you. Remove all malicious code found.
Dump
<iframe src="http://kb.sucuri.net/" width=0 height=0 style="hidden" frameborder=0 marginheight=0 marginwidth=0 scrolling=no></iframe>