Blight has a significantly difficult defense and thus will take a while to defeat him. He will crouch when he does his speed attack, which is impossible to evade. Use the Veil of Mist Brush to slow it down. Blight will lose its color; attack it when he is in this state. He will become a pile of armor with a glowing blue sword. Equip the Tsumugari and use a Veil of Mist Brush to allow more time to attack the sword. Use Excorism Slips and your Tsumugari to attack the sword. Continue doing this to defeat Blight.