What is the ruling on seeking blessings from April rainwater, and has its legitimacy been established in the purified Sunnah of the Prophet?

There is no Shar'i (Islamic legal) evidence to prove any special property for Nissan water, and the righteous predecessors did not do so. Blessed things and those used for healing have been proven by Shar'i evidence, such as honey, Zamzam water, rainwater, and olive oil. Designating Nissan water with blessing or healing without evidence is an innovation (bid'ah). Merely claiming that the Prophet, peace and blessings be upon him, recommended it without a chain of narration (isnad) proves nothing.