Black isn't a sign of marbling. Only marbles with black genes may show irregular black coloring. But ALL marbles of what ever color will turn pale/celophane before it changes back/color up. If I see a betta with irregular fading dark colors and has pale background, I will immediately question if it's a marble. But if it has color (including light colors like yellow or white), I have no idea and will not assume that it is a marble.
So black fins (or black anything) isn't a sign of marbling. There are various (recent) mutations where fins are black or blackish. This isn't due to marbling, though marbles may have them also. Light colors with black scale edges isn't a sign of marbling. It just indicates that it was crossed to black somewhere up it's line.