How to control black spots in the grout lines in a shower?

Did they seal the grout? If not would it help to do so?

https://www.doityourself.com/stry/how-to-clean-mold-from-bathtub-and-shower-tile-grout

You need to find and eliminate the source of the mold spores. Once they settle into your grout, they will just keep reforming. Mold is everywhere, but it needs a source of moisture to survive and grow. I know bleach (sodium hypochlorite) is not environmentally great, but it does work to remove mold spores. Open a window when using. I believe there are safer products out there. Or you might have to dig out all the affected grout and silicone areas by hand if you want a safer fix. Then re-grout and re-caulk with a mold-inhibiting caulk. Keep the grout sealed to prevent it from absorbing moisture (but then some folks think this inhibits proper drying of the material after each use). Do not forget to clean out your floor drain! Take off the grid (hair catcher) and get a bottle brush taped to a longer handle. Use any mold-inhibiting cleaner to scrub out the drain. Keep your shower curtain clean and mold free with frequent washings. If you have glass doors, be diligent in cleaning all the cracks and crevices (why I only use washable shower curtains and removed my glass doors). Dry the shower walls or at least squeegee after each use. Install a high quality vent fan that vents to the outdoors. Good luck with the battle!

black is usually some kind of mold. I would get an old tooth brush and make a paste of baking soda and comet and maybe a little blue dawn. Then scrub the spots that are black. As for maintenance, I don't think dawn dish detergent is what you need because certainly the mold will come back. Look for a product that removes mold from bathrooms and perhaps only use it manually (sparingly) on your grout by hand.

Again, I agree with the science teacher.