Tested hidden backlinks on a couple of experimental domains earlier this year and the results were underwhelming. Hidden links (display:none, off-screen positioning, same-color text) got crawled fine but passed almost no measurable equity - rankings on the test pages barely moved even after 60+ links. The bigger problem is the risk profile: Google treats hidden links as a manual action trigger, and recovering from a manual penalty is way more expensive than whatever short-term boost you might get. My conclusion was that the effort is better spent on contextual placements that read naturally in the content, even if they're lower volume. One visible link from a page that actually gets traffic outperformed dozens of hidden ones in my tests. If you do experiment, isolate it on a throwaway domain and never on a site you care about.