What is the ruling on boiling black grapes and drinking them a day or two later without adding anything to them?

If boiling grapes and drinking them does not cause harm or reach the point of intoxication, then there is no harm in it. However, whatever intoxicates or causes harm is forbidden, based on the saying of the Prophet, peace and blessings be upon him: "There should be neither harm nor reciprocal harm," and "Every intoxicant is khamr (wine), and every intoxicant is forbidden."