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 ASP based, thus intended for server-side use and the payload is executed directly on the server, while the site is rendered. Only the payload result (spam content, redirect) is visible in the browser, not the malicious code itself.
Spam-seo can also be found inside the site's database being injected by ASP malware or other vulnerabilities, like SQL injections or vulnerable CMS.
Any ASP based website, often through vulnerable code or compromised FTP credentials.
Find the files where the spam SEO is injected, the most accessed files are the main target, so look for default.asp, index.html or index.asp. Also, check the database for any signs of injection.