In order for the transmission to vent off excess air pressure the vent has to be open so yes it has to be unblocked.
What I have used in the past is a pipe cleaner soaked in diesel oil to get the muck out of it. Some of the old ones (years ago) were rubber lines that went a short distance and I simply removed the line and cleaned it out with the pipe cleaner trick, some others were a fitting and I cleaned around the fitting and then using a proper sized wrench removed the fitting and then cleaned the entire fitting in diesel oil and then used a dry pipe cleaner in the bore until it was cleaned.
If it is cleaned slowly and carefully you can get the muck and debris out of it without removing but just take your time and if a little dirt does fall in it is not the end of the earth but that is what the filter screen is for and run it for a hour or so then reclean the screen and you should be fine.