I saw this bug when testing a potion on my new minecraft map in the latest snapshot: when you are blind you just don't see the blue lines of the worldborder!
I don't know if it is a bug or just an intended feature, but check out the images...
Also you still see the red grandient around the screen borders. This is too strange for me...
How to reproduce:
Step 1: Go to the worldborder and if you see it's blue lines go to step 2
Step 2: Give yourself blindness
done