No, I do not think that blame could be considered a theme of "The Merchant of Venice." I'm sure Shylock blames Antonio for not being able to pay the loan back or blames Antonio for being friends with Lorenzo. Shylock's daughter, Jessica, ran away to be with Antonio's friend Lorenzo, so it must Antonio's fault right? But a theme is a major point to a story. It's like a lesson that is imparted on the reader or audience, and I just don't see blame being an important enough piece of the "Merchant of Venice."
You are better off picking something like the love of money vs. the love of relationships for the theme. At first glace, Shylock seems to be all about money, but that doesn't run true throughout. He IS upset that his daughter ran out. And when Shylock is presented with the opportunity to be fully repaid and more, he turns it down. Antonio is the similar. He IS a wealthy merchant but at the same time he returns his half of the court "winnings" to Shylock.
Justice could be another major theme of the play too, since most the play is focused on the court proceedings and exactly how justice should be met between Antonio and Shylock.