What is the ruling on taking a loan from a usurious bank to pay blood money (diyya) for an accident, given the impossibility of obtaining the amount from other sources, and what is the ruling on the guarantor in this case?

It is not permissible to borrow with interest (riba) except in cases of dire necessity. Your situation does not appear to involve such necessity, as alternatives exist, such as borrowing from individuals or seeking assistance from charitable organizations, imams of mosques, and tribal chiefs. If the blood money (diyya) is for an unintentional killing, then the 'aqila (male relatives on the father's side) are responsible for bearing it. If you are in financial hardship, the guardians of the victim must grant you a respite, as Allah Almighty says: "And if there is one in hardship, then [let there be] a respite until [a time of] ease."