To make the marinade, combine all of the marinade ingredients in a medium bowl.
Pour the can of black beans into a small pot and add ⅓ of the marinade to the pot. Add the bell peppers and onion to the medium bowl and toss the vegetables in the marinade. Set aside for 30 minutes.
Heat a large skillet over medium heat and add 1 tablespoon of butter. When the butter has melted, add the marinated vegetables. Stir frequently until all of the vegetables are tender.
Heat the small pot with the black beans over medium heat until boiling, then turn off stove.
To prepare your fajitas, place the vegetable mixture in the center of a tortilla, then pour a spoonful of the black bean mixture on top.
Garnish as desired with cilantro, lime juice or jalapeños.