Why would the villain involve Sherlock Holmes?

To answer your question you must understand the way a criminal thinks. It is a known fact about criminal psychology that "Every criminal thinks he is the best and that nobody other than him is as good as him and he would not make a single mistake like others did earlier". This is a wrong self-esteem which often leads criminals the most to their danger. Pride for oneself makes one more vulnerable in one's planning. Often the best brains of Criminals repeat this same mistake. Lord Blackwood no doubt was one of the best brains in his time. But he was no different from other criminals. He too had too much confidence in himself and considered himself undefeatable. In short, this false pride about himself made him challenge Sherlock to catch him.

If you observe carefully, Sherlock has been the only nail in Blackwood's road to success. He was literally the only one who caught Blackwood's tricks. When everybody was calling him Satan or black magician, Sherlock was the only one who revealed his tricks to everyone in the light of logic. So it is a fun for criminals like Blackwood to toy with a tough contender like Sherlock and make him perplexed. As a game does not seem good if the competition is not tough, the same way criminals don't get fun if the Police is not up-to-the-mark. This is the reason Blackwood wanted to involve Sherlock so that he can make Sherlock defeated to him.