Your black tank vent is probably tied into the vertical portion of one of the other vents. This is not a wet vent as Bama Rambler has stated, but the incorrect terminology really doesn’t matter, and I will explain what a wet vent is if you like, but it would not pertain to this anyways. Since the black vent is tied into a vertical portion of another vent, any water you pour down the vent it is tied-in to will drop straight down, past the tee where the black vent ties in and it will flow to the tank which that vent serves.
You say that you have your black tank pumped regularly. With this in mind, if you didn’t have a vent on your tank, the tank would have collapsed the first time it was pumped-out and you would have much more serious problems than what you have described.
Your black tank is vented. No doubt about it. As far as the pressure build-up problem you describe goes, I don’t know what to tell you without actually being there and seeing what is going on. But I can tell you, if I was experiencing something like this, I would start the troubleshooting by slowly filling my blackwater tank with my black water flush system to verify there is some sort of blockage allowing air to enter the tank from a vent, but keeping it from exiting.
2016 Rockwood Windjammer 3029W Diamond
2015 Chevy 3500HD LTZ 6.0 Crew Cab 4x4 Long Bed 4.10:1 SRW