The blind man should guess 50%. The sighted man should guess 66%. So, the blind man has made a good guess, given the information he has, while the sighted man has better information, has made a mess of his reasoning, but still has the more accurate figure.

How to quantify rationality? I don't know. Maybe model the methodology used in IQ tests. If you are comparing rationality across individuals, you should probably make sure they have the same access to the test information - or the results are likely to be screwed.