Im not expert btw, this is just what worked for me.
Step 1. Download "libbluray" and "libaacs"
Step 2. Also download libasm-java, i think its necessary for menus.
Step 3. Put KEYDB.cfg in the folder ~/.config/aacs/
Step 4. I think Openjdk is also needed for menus
Step 5. Play a blu-ray in and enjoy!
Troubleshooting:
If you get an error with the "keydb.cfg", then rename it to "KEYDB.cfg". (note the capitals)
More info about blurays on linux here: forum doom9.
Here's a list of other packages you could install that might help:
"libdvdcss libaacs0 libbluray-bdj libbluray2 libbluray-bin libaacs-dev libbdplus0 libasm-java"