As far as I know it’s not illegal to play a bluray disk on linux. There are instructions on how to do this via makemkv (without ripping the disk) at https://www.maketecheasier.com/play-blu-ray-on-linux/.
makemkv is partially open source except for the actual bluray decompiling bits. If this program was illegal, I expect it would have been shut down years ago, as you can purchase it.