The building front a center here, on Cumberland Ave, is part of an especially dismal stretch of Cumberland Avenue. I believe the blank wall belongs to the Fair Point company (formerly owned by Verizon, formerly New England Telephone & Telegram, formerly Bell, or something like that) so I presume the reason for the blank walls is that the inside is filled with telecommunications equipment (a series of tubes perhaps).
Anyhow, you may be familiar with the Art All Around project by the Maine Center for Creativity, in which several oil tanks in South Portland are being decorated. The artist, Jaime Gili of London, won the competition to have his designs used on the tanks. Here is one of the artist's tank designs applied to this blank wall:
Really spruces it up, eh? I wouldn't want to look at something too crazy if I lived in the Back Bay Tower and my condo looked at this wall, but it wouldn't be worse than a blank wall I think. Feel free to download the above photo and photoshop your own creative design onto this wall. If I had something of value to give, or owned the building, I would make this a competition.