I had a similar problem on Outback about a year ago. No water spraying in the black tank. I do know what caused the problem. It was the anti-siphon vacuum breaker. I attached the garden hose to the trailer and turn it on. I didn't run some water through the hose first to clean out anything that might have crawled up the hose while it sat on the ground. Well ... gunk clogged up the anti-siphon "thing." I tried everything to free it up. You know.... you CANNOT blow backwards into those things! :h
I replaced it (yea, as I recall it was over $60 for that thing), and I've not had any problems since. Now, I always run some water through the hose before attaching.