“Blue Cowboy” is a modest love story, told in a direct, unadorned manner, but it is also complicated, funny, tender and stupendously poignant. In some ways, it’s a relatively predictable account of an attraction that could possibly blossom into much more, if not for daunting odds — Cale drops a “Brokeback Mountain” reference, cheekily pre-empting the inevitable comparison. Yet “Blue Cowboy,” which is masterfully written and just as masterfully performed, never stops surprising.