Australia tried this in the early noughties I believe - running a non-public URL blacklist. After some parliamentary accountability and commmitees got it cracked open, they found that about 10% of the sites met the definition for inclusion, with the remainder being a grab-bag of things various politicians and bureaucrats didn’t like.
Look everyone, it’s the season 37 opener of I’m not going to use the great tool because people I don’t like are also using the great tool!