Much of the tunnel is already there from the time the Blue line was a Streetcar line. Until the early Fifties, there was a portal that came up on Cambridge Street almost at Blossom Street. There was once a streetcar route from Jeffries Point to Harvard Square. But between the former portal and the Red Line MGH stop, there is a ton of underground utilities. Photo of the Blossom Portal is in this article:

http://www.bostonstreetcars.com/the-growth-of-boston-rapid-transit.html