Blight can be defeated fairly easily if you know what to do. The link below explains a good strategy to use against him. It also includes strategy for ALL bosses in Okami.
http://www.gamefaqs.com/ps2/920500-okami/faqs/45114
Blight isn't really that tricky of a boss, this video shows a good method of taking him down, also look in the video description for in depth details on how to beat him.
http://www.youtube.com/watch?v=rXrPCpZxv30