If your raised bed is near enough for a lot of Black Walnut leaves to drop on it, you have more of a problem than just the leaves, sorry to say. The roots of the Black Walnut secrete the nasty stuff that poisons other plants - juglone. So once the roots find your raised bed, you're going to have to resort to growing things that tolerate it anyway.
So, to be honest, I don't see the point of keeping the leaves out with a tarp.
