They do exist, there not mentioned on the board because there not well recognized. But basically a Blister is a tunnel like formation where unless your specifically targeting the sectors E-probes will not likely warp through there.
They also do not show up as a Dead end or a bubble on a ztm.
They are warp irregularities that share the same entrance and exit. As such they can make ideal hiding spots, however many of the long time players know about them and usually will target them first. Since many bangs have less than a handful of "true" blisters there extremely easy to flush players out of.
Back in the day before they were well known they were the shiz.
//The simplest way to visualize a blister is a tri-force, yes the Zelda game.
For a blister to exist both the entrance and the exit are generally a single sector. This can expand out beyond 3 unique sectors but tends to be rarer than the standard three sector Blister.
Maybe Kav or EP can weigh in and give a better description.