Only some explosions can be blocked: for example, creeper explosions and TNT set by other players is blockable, but TNT ignited by redstone cannot be blocked.

Only some explosions can be blocked: for example, creeper explosions and TNT set by other players is blockable, but TNT ignited by redstone cannot be blocked. Explosions from TNT set by the player himself also cannot be blocked, but that's due to MC-92017 rather than this bug.

This can be tested easily enough by seeing if the shield loses durability or not from the explosion.