Gain Block before attacking
Body Slam turns your current Block into attack damage. It does not create Block for itself. If your hand contains the attack and several defensive cards, play order determines the Block value available when Body Slam resolves.
Its upgrade lowers the energy cost from one to zero. That can allow the same defensive turn to finish with an attack rather than making you choose between another Block card and dealing damage. A zero-cost copy still needs enough Block to make that attack worthwhile.
The printed effect does not spend all your Block as a cost. However, Block is still a resource enemies can remove, and ordinary Block does not persist across turns without an effect that preserves it. Owning Body Slam is not evidence that the deck has a working defensive engine.
Compare Barricade with the cards that actually generate Block: keeping Block and producing it solve different problems. Before choosing a Smith, inspect whether a cheaper Body Slam creates an additional useful play in the upcoming fight. The Rest versus upgrade guide explains that comparison without assuming every cost reduction prevents the same amount of damage.