BlackRock CEO sees 'more to go on the upside' for the stock market

"With interest rates as low as they are, we're seeing more and more investors focusing on where they put their money," BlackRock CEO Larry Fink told CNBC's "Squawk Box" Tuesday. "We have a strong conviction that the average investor still is under invested and they're going to have to be putting more and more money to work over the coming months and maybe even years."