For this scenario, I'll use V for vision to describe the alleles. The cross would be (Xv Xv) x (XV y) giving a 50% of each gender. The way you asked the question is asking for the probability of the child being colorblind given it is a boy, and the answer is 100% because in each case a son would inherit the y from the father, and then a recessive Xv colorblind allele from the mom. In you want the probability of the child being colorblind and a boy out of the total possibilities it is 50% because all the sons will be colorblind.
