I used to be a shampoo assistant and had to wear all black. It's a difficult balance since you need to look put together and covered up, but it's an active and demanding job and you will sweat a ton. My #1 tip is not to buy expensive black clothes. You will get bleach, hair dye, and other assorted chemicals on them, on top of getting above average wear & tear.

My go to-combinations were leggings with a black tunic, a black skater dress, and high waisted black jeans with a sleeveless collared shirt.

I would stay away from maxi dresses. You will have to be nimble in this job and a maxi dress will catch on things.