The blast protection enchantment does not affect damage or knockback from ravager roars.

To reproduce:

1. Enchant a mob head with blast protection IV in creative mode.
2. Obtain a shield and a ravager spawn egg.
3. Go into survival, blocking the ravager's attacks until it gets stunned.
4. Stand in front of the ravager until it roars, noticing the damage and knockback of the roar.
5. Repeat steps 3 and 4 with the mob head equipped, noting that damage is the same. After multiple tries, it'll also be noticeable that knockback has the same potential with or without the mob head.