How is bill blaming willy for his failures?

I think you mean Biff. Biff was emotionally torn by his father's affair, and the two of them never worked it out..... his father was a poor example, and possibly, Biff uses his father's mistakes as an excuse for his own lack of success. Biff had looked up to Willy at one time and followed his father's example but eventually realized that his father is a failure at life