Leaf litter can be a great natural groundcover or mulch material. Shredded leaves may be easier to spread and look more tidy, but whole intact leaves is more ecologically beneficial, in that it provides more protection to organisms using it for shelter or food. (The shredding process could injure or kill insects like moth or butterfly caterpillars or cocoons sheltering the leaves.) Leaf mulch depth varies based on the tolerances of the plants it's covering (woodland plants typically are fine with thicker layers than meadow plants) and how quickly the litter breaks down, which can depend on what tree leaves it includes. In general, several inches for ornamental plant beds should be fine; over a foot (since it will degrade greatly from autumn to spring) can have the excess removed and used elsewhere, or composted.

Leaf litter mulch can be used around vegetables as well, and if shredded, a few inches (maybe 3 or so) should be fine. We are not aware of any tree or shrub leaves used as a leaf litter mulch that would risk harm to vegetable (or other) plants. While Black Walnut foliage, nuts, and roots can all produce a compound (juglone) that can potentially stunt or interfere with plant growth, not all plants are sensitive to it, and in a situation where its leaves are mixed-in with other tree species and not used by themselves, it's doubtful any substantial impact would be observed. Some lists of plants that are apparently tolerant to juglone are compiled by various Universities, though they don't always agree with each other and overlap completely. This may be because the impacts of juglone need more study, since laboratory observations doesn't always translate to "real-world" impacts in the environment where many other factors interact with or interfere with juglone.

Miri

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