That sounds like an edema, I've had one on one of my orchids in the past.
It probably was full of liquid initially but has burst. You are right it's just like a blister.
I was told they can develop if the plant takes up more water than it can then cope with... the roots are doing their job a little too well
I don't think it's a problem unless an infection gets in after the blister bursts. On mine I put cinnamon on the inside after it burst. This dries it out and helps avoid infection.