Well, if you insist on metaphysical reasons, then you may wish to switch the prayer you use. Understand that a blessing is merely a well wishing. It is not truly "holy". What makes it "holy" is your reverence to the sacred idea that it represents. "Holy water" is generally believed to be blessed by a particular deity. If you do not share the view that it is associated with something that you consider sacred, then it will react in whatever way you actually associate it with. It could be considered psychosomatic in this respect as your subconscious can become very empowered with focused ritual. This is why it is very important in magick to Know Thyself (Oracle of Delphi) truly. If you do not understand what you find to be truly sacred, and utilize what you are actually uncomfortable with, then it will have a negative impact instead of a positive one. This same thing could occur in spells. If you cast a love spell but have a fear of commitment, it can "backfire" through your fears, despite the intentions being clear.
This is why Paul Hudson suggests the ever famous recitation of the Lords Prayer backwards in order to mentally break free of the fears that Christianity can create such as the concepts of sin and hell. Practicing witchcraft while holding onto such concepts can become very destructive to the self and the Self.