No, a katana cannot effectively block a bullet.

Katana swords are designed to cut through or slice objects with their sharp, curved blades. They are not designed or built to withstand the force and velocity of a bullet. The speed and impact of a bullet far exceed the capabilities of a katana to stop or deflect it. Attempting to block a bullet with a katana would likely result in the bullet penetrating the blade and causing significant damage or injury to the person attempting the block.

When it comes to defense against bullets, it is important to rely on appropriate and effective modern ballistic protection, such as body armor, shields, or other specialized equipment designed for that purpose. These items are specifically engineered to stop or mitigate the impact of bullets and offer better protection than traditional bladed weapons like the katana.