First a huge thank you to KI for listening to the decor community and making castle blocks with no red trim.
However, I've been trying to apply papers to the Regal Wall Castle Block B and they don't seem to stick well. I had a lot of trouble getting the second side to take a paper (the brick side, I was putting the paper on the brown side first). Sometimes with fiddling I can't even clearly describe, I can get the block to accept them both and everything looks good, but if I leave the house and return, the block no longer has the papers on it. If I log out and then return, same thing. This happened whether I put the same paper on both sides or whether I had two different papers.
Also, I decided to try removing the papers from the house paper library and this interface erroneously removed a bunch of papers (of the same type) from the actual walls of my house.
Note the the block I am using does have some castle magic grows on it and I also did advance move it into its current position, although I applied the papers after doing all that. Not sure whether either of these things are related to the problem--I haven't had time to do very extensive tests with these blocks.
I hope these issues can be solved, because I have many ideas for them--these trim-less wall blocks are exactly what many decorators have been dreaming of for years. Thanks! :)