How can I clean mildew off of concrete?

Clorox and baking soda

Bleach is wonderful to clean mildew

First kill the mildew with a solution of Bleach and water in a spray bottle. Try 6 parts water and 2 parts bleach. Wear old clothes. Let dry and clean off cement with water. Not sure this will remove the black but it will kill the mildew. If it removes the black stain, suggest you seal the cement when it is clean, a solution of Simple Green will clean a lot of tough stains. Hope this helps. Oh yeah, you should wear a mask In case you are sensitive to the bleach smell.

I'd scrub the cement using a bleach solution. You may want to leave it on for a while before rinsing off with a strong spray.

Hydrogen peroxide will work, just do not get it on grass or plants. Pour it on and let it sit.

Would you let me know how it works out please?