After grouping the metal blocks into 3 groups of 3, the three groups can be named A, B, & C.

Now A and B is weighed

Case 1: A is lighter => A has the lighter block

Case 2: B is lighter => B has the lighter block

Case 3: Balanced => C has the lighter block

In any case weighing 2 of the 3 from either A, B, & C gives further 2 cases

Case 1: Either one goes up or down => The one that goes up is the lighter one.

Case 2: Balanced => the other block is lighter.

So a minimum of 2 steps is sufficient to identify the lighter block of metal.