I've been looking for the source that the famine actually ended when Yaakov blessed Pharaoh. I found it in the Midrash, but I'm not sure whether it is a purely midrashic source, or whether it is based on the Gemara or some other source. I know Rashi says that what the blessing was actually that the Nile would overflow, but Rashi isn't known for citing his sources and this is no exception.
It is pretty clear that the famine was supposed to last seven years according to the text; there are some say that it was supposed to last for >40 years. Was the blessing even to end the famine (given that the next perek talks about the rest of Egypt paying for their food)?