It does not necessarily follow that raising taxes on vacant land will spur development. It just creates a disincentive to owning land. How that disincentive affects the owner's use of the property depends wholly on the development potential of that parcel. It might motivate speculators to either develop or sell to a developer in higher value areas, but in distressed and transitional neighborhoods it isn't going to spur development where there are no market forces demanding it. It merely reduces the market value of that parcel to account for the added cost of owning it.
Blight is best dealt with by fining safety violations and holding property owners liable for the maintenance of their property. L&I's doors and windows enforcement is a good start. Someone owning vacant property in an area not yet ready to support redevelopment shouldn't be punished simply for owning that property so long as they properly maintain it.