The first line of the perk “increases flame damage of mounted weapons” is the mention you are looking for. Tempuras and Tachi do count as flame weapons, and Blight will give you 20% damage buff, and more importantly, cool green flames.
I recently bought a Blight and put it on my Tempura build, which used to be built around a Torero, because accuracy is important with melee, amirite? I wish it made a big difference, but at low-mid PS it really doesn’t. You either hit something very crunchy and the opponent dies, or you don’t and you get stuck. Green flames are cool and all, but I think I liked disrespecting my opponents with a melee Torero build more.