Yep, they block them 100%. You won't get set on fire or take any damage whatsoever. If you don't want to waste your Blaze rods on fire resistance potions, just keep your shield up, and facing the attacking blaze and your good to go.

Do shields block ghast fireballs?

When a ghast fireball is blocked by a shield, the fireball itself is blocked, but the explosion still damages the player. Expected is for the explosion to be blocked by the shield, like creeper explosions.

When was sword blocking removed?

This is a feature that was removed from Minecraft Java Edition in version 1.9 and I don't think it was ever in Bedrock Edition. It should be re added in this new combat update. It would work mostly like how it works on the legacy console versions.