I am a retired Science teacher, with a minor in Chemistry, with enough knowledge to be dangerous but not enough to be an expert. I also live in rainy Western Washington and have had issues with rust in firearms and barrels. Please feel free to correct me if you know more.
I have found that it is possible that oily cleaners like Butch's may not themselves cause the damage, but the ammonia in Butch's causes copper metal Cu(s) from jacketed bullets reacts with the ammonia and forms the corrosive ion Cu+2, which has the blue color, that shows up on your patches. If these active ions are not completely removed in cleaning, they will grab electrons from iron in barrel steel and the iron will then be vulnerable to oxygen, causing rust. So to translate. Clean naked iron may not be rusted by Butch's but if copper is present, even in tiny, tiny amounts, the resulting reaction will result in a rusted barrel.