TheBostonian identified it. It's the Tufte Center, Emerson College's only completely new, purpose-built structure. You can see it from only one viewpoint: the small alley running off Tremont Street, between the Majestic Theatre and the Little Building. It's really jammed in there between the Majestic, the Colonial, and the Transportation Building. The best time to view it is at night, when it's lit up in constantly changing colors.
The public entrance to the Tufte is not from that alley, however. It's from Boylston Place.