It should be a B vent, double wall, going thru an attic space. 6” clearance around combustible on a C vent, single wall. That to me, doesn’t look like charring. You could paint that area and keep an eye on it to see if it discolors again. Was this here maybe before you got the place? Maybe previously the vent didn’t extend all the way up to the roof Jack and was resolved but the discoloration was never bothered with?