No.
The blunderbuss fires in a 15-foot cone when firing pellets, and has a 10-foot range increment when firing a bullet. A blunderbuss uses a bullet or pellets and a single dose of black powder or a single alchemical cartridge as ammunition.
So it fires a cone OR fires a bullet. One or the other. Only the bullet has a range increment; the cone does not. Instead of a range increment, the cone has an area effect. If you want a larger area effect, you would need an ability that enlarges the area. "Distance" does not enlarge an area.