Yep...the vent IS blocked. And yes, you should definitely be concerned with pressure buildup in the system, 'cuz that can result in all kinds of nasty things...from a geyser when you open the deck
pumpout fitting or an eruption in your face back through the toilet (It's gonna relieve the pressure wherever it can), to a burst tank. A blocked vent won't allow the tank to be pumped out either...'cuz without a source of air to replace tank contents as they're pulled out, the pump will pull a vacuum that prevents the pump from getting more than a gallon or two, if even that much. A particularly strong pumpout can even implode the tank.
So do NOT use the toilet again or attempt to pump out or dump the tank till you're sure the vent is open.
The two most common locations for a for vent line blockage are the vent thru-hull and the vent line connection to the tank--both the tank fitting and that end of the vent line. So start by scraping out the thru-hull with a screwdriver blade or whatever works. If there's a screen in it, knock it out...screens cause more problems than they solve. Then go to the tank and remove the vent line from it...and scrape out the tank fitting and that end of the hose.
If there's a filer in the vent line, get rid of it...'cuz it's gotten wet inside and getting wet ruins it...and also causes the charcoal to swell and block the vent. Vent filters are a horrible idea on any boat...worse on sailboats because heeling causes tank content to spill out the vent, destroying the filter and causing all kinds of problems. They also cost an arm and a leg...so if there is one, get rid of it!
Trace your vent line to make sure there aren't any kinks in it...or any sags where liquid can pool..'cuz pooled liquids in the vent create a blockage. If you find any kinks or sags, straighten out the vent and get rid of both.
Now that you've succeeded in clearing any blockage, you don't want another one...so to prevent it, backflush the vent line with the hose every time you wash the boat and/or pumpout. If the thru-hull won't let you do that, replace the thru-hull with one that will.
And, install a gauge. The Profile gauges Profile Tank Monitors
are the best "bang for the buck"...they tell you how much is in the tank, not just when it's full...and also have the best customer service
on the planet!