Yalkut Yosef (432:1) specifically paskens that normally we do not say a beracha when we are in doubt whether or not we are about to actually perform a mitzva, but in this case we do recite the beracha on the search even if we do not know if we will find chametz.
Note, he also says that we do not say the beracha specifically about the act of searching, because it is only a step leading to the goal of getting rid of the chametz.