Google has done a lot to try to reduce the amount of spam created to game the algorithms used to generate the organic search results of their search engine. Some of these actions have been great, while others have had unforeseen repercussions.
One of the biggest changes they’ve put into practice is their attempt to quality check the links being built. Links theoretically work as a kind of ‘vote’ in favour of the website being linked to; companies artificially built up links by placing hundreds of links on a variety of low quality websites, which were clearly built up for the sole purpose of providing links to paying customers, which google felt was causing damage to the results. They instigated an update which targets these websites, which then causes these websites to convey a negative vote. This means that any link from one of these blacklisted websites will damage your website.
Negative SEO is where somebody gets links to a website put onto a blacklisted website, so that they move down the search results. This has occurred as a direct result of the updates google has put into place.