"Not every site that has had its url spammed across forums and in comment-spam did it themselves. The other side of black hat SEO is destroying competitors by t

"Not every site that has had its url spammed across forums and in comment-spam did it themselves. The other side of black hat SEO is destroying competitors by triggering Google penalties against them." That tactic (of spammy linking to your competitors) only became a viable one after Panda/Penguin. I doubt it was practiced by SEO agencies before those algorithm changes. So links added before that change are unlikely to be negative SEO.