I can add that if you have this problem, blowing through that tube is a total waste of time, even using compressed air? Spider webs are much stronger than most people realize! The burner tube really needs to be removed to be cleaned. This also allows a good visual inspection to confirm it's clear prior to giving it another try. Saves a lot of messing around.
That black stain is almost always attributed to a plugged main burner tube.
From the 'lessons learned the hard way' manual developed in my old RV tech days....
BTW - this problem not that unusual in gas fired barbeques as well!
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake