Brit. slang especially prevalent in the Jeeves stories by P.G. Wodehouse. An annoying person. Perhaps the word is related to "blight", a plant disease that ruins potatoes (as a "blighter" tends to ruin things in which he's involved). The word appears to be milder in force than "bastard".
I tackled the blighter squarely. I mean to say, nobody knows better than I do that Jeeves is a master mind and all that, but, dash it, a fellow must call his soul his own. -- from The Inimitable Jeeves, by P.G. Wodehouse