777 and all APP powders are hygroscopic, this is a known fact. The APP powders are worse than 777 but all powders that are hygroscopic become erratic when they absorb a little moisture out of the air. Most will still fire but usually the first thing to go is accuracy.
I have some Pyrodex that is 20 years old and it fires just fine but I think it chemically aged some as the groups are an inch to two inches bigger.
According to the manufactures Blackhorn is the one supposed to have a long shelf life.
I know water does not affect it much as I ran some experiments wetting it and drying it and it worked just fine.