Blight is the term for the physical corruption of the land by the presence of the Lich King's undead plague. The plague affects all it touches, sterilizing the land, warping its natural inhabitants into bloodthirsty mutants, and twisting the local flora into disgusting mockeries of proper plants. The Eastern Plaguelands (most specifically Plaguewood) and the Dead Scar that runs through Quel'Thalas are perhaps the most startling examples of the long term effects of Blight.WotLK This section concerns content exclusive to Wrath of the Lich King.
The lower levels of Zul'Drak are under siege by three necropoli. In addition the Scourge is making use of the blight to turn these areas into sanctuaries for the undead. Their intention is to corrupt the entire zone.Pilt:WC3RoC logo 16x32.png This section concerns content exclusive to Warcraft III.
In Warcraft III, the blight is a core mechanic of the Undead race. Players are only be able to build undead structures on blight (with a few exceptions). Whenever a building is finished, it instantly creates another layer of blight arround it. Furthermore, undead creatures regenerate faster while on blighted soil.