What is the evidence from the Qur’an or Sunnah against the impermissibility of seeking blessings from the Prophet (peace be upon him) after his death, and is the Hadith of Al-Abbas (may Allah be pleased with him) regarding the prayer for rain considered evidence for that?

Intercession (Tawassul) is of two types: a permissible type, which is that done through faith in Allah and obedience to Him, and through the prayer and intercession of the Prophet during his lifetime, and intercession through the beautiful names and sublime attributes of Allah, and through righteous deeds. And a forbidden type, which is intercession through the person of the Prophet after his death, or swearing by Allah through his person, or intercession through graves and the dead, because this involves polytheism (Shirk) and the prohibition of sound hadiths that forbid taking graves as places of worship, such as the hadith of Aisha in Sahih: "Indeed, those who were before you used to take graves as places of worship. Do not, then, take graves as places of worship."