-How do you train for them?
Pretty much the way that you have explained in your materials. Poison birds must be trained in a logical fashion, otherwise the dog will be confused as to what you are asking it to do. It's a step by step process. Once the dog indicates an understanding of the concept, it's important to let it get the mark after it has picked the blind ... unless he showed too much interest in the mark while running the blind.
-Why use them?
From a judging point of view, it's a test of control ... which is what a blind is about. From a participant's point of view, you must have this degree of control to win field trials. Although there are some judges that abhor poison bird blinds, they are in the minority. (Frankly, I believe that the reason some of the judges hate poison birds is because they have or have had a dog that couldn't be pulled off of one.)
-How frequently? in training, in tests/trials
In training: With my first truly competitive dog, for a time we trained on PB blinds about once a week, sometimes twice. During that time, our dogs were exceedingly proficient in running these blinds ... they understood that if they ignored the mark while running the blind, they would eventually get the mark. The dogs understood it so well that they would run an under-the-arc blind without a whistle ... that's a thing of beauty. When they get too cocky, you can run double blinds, then pick up the mark ... that is a real killer at first, especially if the second blind is in front of the gunner and tight to the mark. We only do that with advanced dogs.
In trials: In Opens and Ams, PB blinds show up about every other weekend in my FT circuit.
-What are the judges looking for? in tests/trials
Control. And, if they have the dogs eventually pick up the PB, it tests for memory, as well as courage to go into an area which was earlier off limits.
-How important a skill for a hunting dog?
Very ... they have to get the cripple that hit the water 120 yards away before picking up the dead bird lying in the decoys at 30 yards away.
-What constitutes a failure? in tests/trials
Of course, picking up the PB is a failure. Beyond that, it's up to the judges; however, a demonstration of lack of control (i.e., too many whistles/cast refusals to pull them off the bird) will send you home. Also, sending the dog on a line so far away from the PB that it is not inviting to the dog will be frowned upon (some would call this a failure to "challenge the blind," but I dislike cliches so pretend that I didn't say it).