"At the top of that panel, there's a section called Storage. If you click that, one of the types of storage you can inspect is cookies (at the left of the panel). The NSFW opt-in status cookie is quite clearly named. I just deleted it, and I have the blurred images and opt-in buttons back. "
I tried this procedure and it worked. However, once I left the site and came back the images are back to being greyed out, and no Opt-In option.