Priesthood Blessing
"Brethren, our task is to reach out to those who, for whatever reason, are in need of our help. Our challenge is not insurmountable. We are on the Lord's errand, and therefore we are entitled to the Lord's help. . .
"Ours is the responsibility to so conduct our lives that when the call comes to provide a priesthood blessing or to assist in any way, we are worthy to do so. We have been told that truly we cannot escape the effect of our personal influence. We must be certain that our influence is positive and uplifting."
(Thomas S. Monson, "True to Our Priesthood Trust," Ensign, Nov. 2006, 58)
Of course, because it is by virtue of the Faith of the one receiving it that the blessing/ordinance is obtained. First of all, in most instances it is really impossible to know the worthiness of someone giving you a blessing. One wants to assume that every Priesthood holder who performs an ordinance or gives a blessing is worthy to do it, but the reality is different, people commit mistakes and many live double lives or haven't been aware that need to repent. It would be unfair if the Lord would stop his needy children from getting a blessing just because the performer is unworthy of the Priesthood.