Jeff,
You might have the same problem I do. I don't have good feedback from my body when it starts doing something wrong (boy, that could be freudian, couldn't it? but we'll stick to archery ;-)). My current slump was because I had dropped my back tension a little bit.
The blind bale work probably came about because other archers also have some trouble self-identifying problems with their form. Shooting into a big target from a few feet away means that none of your mental focus is directed on hitting the target, and 100% of your mental focus is directed on your form. I know that even Olympic archers do this on a regular basis.
When I attended the instinctive shooting clinic put on my Fred Asbell and Ken Beck, all we did during the day was alternate short periods of shooting into a bale a few feet in front of us with other periods where Fred or Ken would discuss an element of form. So you had one item to focus on during the shooting sessions. It got to be pretty boring shooting into the bales all day, but they had other bales out in the field we could have fun shooting at when class was not in session. But boring or not, it was an effective way to learn the fundamentals of good form.
As I said, though, having learned the fundamentals once doesn't mean you're going to keep them forever.
I'm not sure what good it would do to shoot into the blind bale if you've never learned good form to start with. I spent several years before the class reading everything I could get my hands on about archery form, and my self-teaching got me about 95% of the way there. Fred and Ken pointed out a couple of things to me that I was unaware of that improved my form. Everybody in the class seemed to be in about the same boat. It gave me some added confidence knowing two really qualified people had looked at my shooting and found that it was basically pretty good, with a couple of minor adjustments.
It might be a good idea, if you don't have someone handy to check your form, to make a video and post it on this forum. But if you already have been taught good form, sometimes all it takes it a nudge in the right direction, like Rod gave me above.