Main Keyword Penalties

So has anyone experience this, lets say we have a site selling black hats. My main keyword was ranking 3rd for "black hats" and loads of other keywords, including 1st for "buy black hats".

Now my site got mega-f***ed by G last month and is at rank #100 right now for "black hats", but only 3th for "buy black hats". I know it is a penalty, I just can't figure out why.

My link building is all the standard stuff: blog comments, web 2.0, 30-40 off homepage-pbn, 4 homepage links, some custom…

Main Keyword Penalties