The only thing that make sense is: <font color="white"> </font> 18 of the monks are blind. Thats the only thing which fit with the statemaent: a stranger says some are BEMs. the starnger comes with new info, the alarm goes off and two monks that are not blind will each know that they both are blue-eyed because the stranger said "some"

Alternative: 19 are blind. last one is blue-eyed. The word "some" is not taken into consideration.