Praise be to Allah.
Blowing into water is of two types:
1 – If this is done with the intention of seeking barakah (blessing) from the saliva of the one who blows into it, this is undoubtedly haraam, because the saliva of any person is not a means of blessing and healing, and there is no one from whose relics blessing may be sought except Muhammad (peace and blessings of Allaah be upon him). Blessing cannot be sought from the relics of anyone else. Blessing was sought from the relics of the Prophet (peace and blessings of Allaah be upon him) during his life and after his death, if those relics remained. Umm Salaamah (may Allaah be pleased with her) had a small bell-shaped vessel of silver in which she kept some hairs of the Prophet (peace and blessings of Allaah be upon him), from which the sick sought healing. If a sick person came to her she would pour some water over these hairs and stir it, then give it to the sick person. But apart from the Prophet (peace and blessings of Allaah be upon him), it is not permissible to seek blessing from the saliva of anyone, or from his sweat or his garment, etc. Rather this is haraam and is a kind of shirk. If the blowing into the water is done for the sake of seeking blessings from the saliva of the one who blows on it, then it is haraam and is a kind of shirk. That is because everyone who attributes an effect to something with no evidence from sharee’ah or proof in reality, has committed a kind of shirk, because he is making his blowing as a cause of healing alongside Allaah, and proving that something is a cause of something else is to be based on shar’i evidence. Everyone who follows a means that Allaah has not made to be a cause, either according to reality or sharee’ah, has committed a kind of shirk.
2 – If a person blows with saliva after reciting Qur’aan, such as al-Faatihah – which is a kind of ruqyah and is the greatest kind of ruqyah that may be recited over a sick person – so he recites al-Faatihah then blows into the water – there is nothing wrong with this. Some of the salaf did this, and it is effective and beneficial by Allaah’s leave. The Prophet (peace and blessings of Allaah be upon him) used to blow into his hands when going to sleep, after reciting Qul Huwa Allaahu Ahad, and Qul A’oodhu bi Rabbi’l-Falaq and Qul ‘A’oodhu bi Rabbi’l-Naas, then he would wipe his hands over his face and whatever he could of his body. And Allaah is the Source of strength.