@alisha-- I used to get those a lot too and still get them occasionally. It doesn't contain any pus and looks more like a bump than a blister right?
My eye doctor told me that these are caused by irritation or infection. Women tend to experience it a lot due to makeup. Old or dirty makeup accumulates bacteria and causes these blisters on the waterlines.
Treatment is very easy though. Just put a drop or two of baby shampoo in clean hot water (boiled water). Wait a little bit so that you don't burn yourself and then dip a cotton ball in it. Use the cotton ball to clean your eyelashes and waterline.
I usually close my eyes and keep the hot cotton ball on my eyes for a while and then wipe. Do this everyday for a few days and the blisters will go away. Also, throw away your old and dirty makeup.