An example of why this is incorrrect.

If a card is the ace of spades, it is black.

A card is black if and only if it is the ace of spades.

There are other conditions under which B (a card is black) can happen, so the second statement is not true.

A conclusion that would be correct is “If a card is not black, it is not the ace of spades.”. The condition is that if A is true B will also always be true, so if B is false we can be sure that A is false as well - i.e. “If not B, not A”.