Maybe cabbage moths / larvae. They will lay eggs on the leaves and then when the larvae hatch, will devour the leaves. Look on the underside of the leaves - if you see patches of greenish eggs, smoosh them or spray with a hose. That's what I do with mine. You can also cover the plants with netting or row covers to prevent the cabbage moths from laying eggs.

Spray with BT and it will go away.

I second the BT. If you look closely on the leave's undersides, you'll likely find a green caterpillar. They are hard to see cause they have excellent camouflage.
Encourage wasps in your garden.. I have lots and they help keep the caterpillars under control. I can see the wasps on patrol, look around my plants for these.

Black stuff is caterpillar poop.

As has been said, look for the caterpillars. Then deal with them.

Bt is your best bet. It is organic, it works well and best of all, only targets the caterpillars not the good guys. I use the Garden Safe Bt Worm & Caterpillar Killer, but I have to order it online, since nobody around us seems to carry it.