We are in the process of building a home in Florida and the exterior walls are of cinder block, to be covered with stucco. They have just put up the exterior walls with steel rod reinforcements and then they put cement, from the top down, into the cement block openings. In two places, down low on the walls, the cement block blewout, for want of a better word. The block contractor covered the hole with a board until it dried and said not to worry, but I am.

Wet concrete is enormously heavy, and if the walls were full height when they filled them, it's not surprising that the bottom blocks "blew out". The pressure at that depth would be considerable. Since the concrete is filling the places where pieces of block blew out, I agree with your contractor...there is nothing to worry about.