AM I BLUE? Whatever you do, don't tell the little ones:  That snide, discourteous lout selling lattes in TV spots for Dunkin' Donuts' new Cafe Blend is actually STEVE BURNS (inset), the gentle, perma-smiley "Steve" from Nickelodeon's kiddie sensation Blue's Clues.  "It was great being the anti-Steve, even for one day," says Burns, who also stretched his dramatic muscles playing a nerdy murderer on an episode of Homicide last season.  Burns says "Nickelodeon has been wonderful" about allowing him freedom in other parts.  And he's not concerned hat he might disillusion his tiny fans by playing more adult roles.  "I would feel terrible if I thought that was a danger," says Burns, adding that he thought "little kids would never make the connection."  Well, most little kids.  Maura Rearden, Dunkin' Donuts' director of advertising, says that the 2-year-old grand-daughter of a former CEO saw the Cafe Blend ads, recognized Burns, and ended up with an autograph.