Blackhat 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 PHP based, thus intended for server-side use and the payload is executed directly on the server, while the site is loaded. Only the payload result (such as malicious iframe, or redirect) is visible in the browser, not the malicious code itself.
Any vulnerable PHP based website. Outdated software or compromised passwords can act as an infection vector.
Cleanup is done by deleting the malicious code from the file, or replacing it with a fresh version. The infection can be found in your system by searching for the malicious string inside your files.