Well, of course no. watching lighting can not lead to blindness. So you do not need to worry about it. As we know that it's only the ultra-violet light that damages the retina, and then leads to blindness. And lightning can just radiate some, but its duration is short, you are not too close, so it can be fine. But on the other hand, you are not supposed to watch it too long, for it also can be bad for your eyes in a long time. By the way, Arc welding is much more dangerous to lead to blindness, for you're closer and it lasts a significant period of time. Anyway, just be careful about it.
Yes, it is true that watching lightning will be bad for your eyes which may cause your blindness. When you watch near the light resource, your visual nerves will be damaged at the sudden lighting situation which may cause the blind spot. You should not do this often. Once you find something wrong with your eyes, you'd better go to the hospital as soon as possible.
I once heard of this saying too when I was little. I am sure watching it is very bad to eyes. It is easy to understand. Eye works like camera. And pupil is the aperture. It controls the light entering retina. When the environment is dark and light is dim, pupil will dilate to let in more light so that it is easier for us to see. Yet there is a special part in eye, which needs to take a while to be active. This is what we said we need eyes get used to the dark environment. When this part is active, we can feel easier to see clear in dark. But if we watching the lightening, our pupil is still dilated, it can't adjust in time, and then too much strong light will get into retina, which could damage retina. It is better not to watching lightening just like you can't stare at the sun directly. if you keep doing this, sooner or later, you might lose your vision. Eyes are very delicate organ, so we need to protect them carefully.