Yes, it is in that form.
Ok. I’ve done a bit more experimenting, and the issue only shows up for domains which have already had a link preview generated. So, if you create a new post, paste in a link to domain X, which generates a link preview box, (don’t even complete the post) then block the domain, then create a new post, paste in the same link, the link preview is generated despite it being blocked.
Additional info: version I tested on: Discourse 2.9.0.beta7 - GitHub - discourse/discourse: A platform for community discussion. Free, open, simple. · GitHub version 5894e7d30f2c597b9750f6b6a55d81f9d101c037