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 HTML based, thus intended for client-side use and the payload is executed after the infected page is loaded directly in the browser.
Any website hosting vulnerable software or server which had the credentials compromised.
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.