The short answer is that it's not just about getting close to their target but also not tipping off passers-by about their identity. Given how noticeable they are (in terms of there being three of them, as well as their unusual racial mix), pretending to be blind allows them to travel to Richmond Road without attracting unwarranted attention.
Just before six-fifteen, the silence of Richmond Road was softly
broken. Three blind beggars came round the corner of the intersection
and moved slowly down the pavement towards the four cars. They were
Chigroes—Chinese Negroes—bulky men, but bowed as they shuffled along,
tapping at the kerb with their white sticks. They walked in file. The
first man, who wore blue glasses and could presumably see better than
the others, walked in front holding a tin cup against the crook of the
stick in his left hand. The right hand of the second man rested on his
shoulder and the right hand of the third on the shoulder of the
second. The eyes of the second and third men were shut. The three men
were dressed in rags and wore dirty jippa-jappa baseball caps with
long peaks. They said nothing and no noise came from them except the
soft tapping of their sticks as they came slowly down the shadowed
pavement towards the group of cars.
The three blind men would not have been incongruous in Kingston, where
there are many diseased people on the streets, but, in this quiet rich
empty street, they made an unpleasant impression. And it was odd that
they should all be Chinese Negroes. This is not a common mixture of
- Dr No. Ian Fleming
Let us imagine that Strangway or one of his associates were to pass these three gentlemen walking along normally on their way to the club, then to see them a few minutes later pretending to be blind. One can envision that that would instantly cause suspicion.