Blackhat SEO is a malicious technique used to manipulate the search engine results in order to benefit a website in terms of relevance.
Any vulnerable website. Outdated software or compromised passwords can act as an infection vector.
Inspect your files looking for the hidden iframes or any code that you do not recognize. Also, you can contact Sucuri to help you with the infection removal.
slightly obfuscated JS code which purpose is to hide this page element: