My Dad's 91 Ranger 2.3l 4x2 that was supposed to be my first vehicle had this exact same Decal, and it was black. I wonder if it was an OEM option? like an early versio of the Spalsh package.
Edit: the closest thing I could find was the 1991 Ranger Sport package which included the blacked out front enda and decal package. the only picks I could find showed it in a much uglier lime green, and blue.