Love is blind is mostly a case of the way others see an affair of the heart.
Sometimes it can be as simple as others thinking that he could do so much better than her as perhaps she's not particularly pretty. Shallow, I know, but with many, it's an important part of how they judge.
Sometimes it can be how one partner behaves and onlookers may feel that there's no way in hell that they would put up with what the other is having to contend with.
For both, onlookers may see their situations as a case of love being blind, but that's the thing with love, isn't it?
What's right for one may not be right for another.