I live just south of Boston (Rhode Island, to be exact) and I go often decked in Jays gear. I've had mixed experiences.
Most of the time, if the Jays are winning, I'll get chirped at by fans sitting around me. I'm not one to be obnoxious when I'm in "enemy territory," so most of it is unprovoked. However, if the Red Sux are winning, they don't say much of anything to me.
Besides Fenway, I've seen the Jays at Yankee Stadium and it was a much more pleasant experience! This is even when it was a game the Jays had won! Other than that, I haven't seen them at another opposing team's stadium.