Are you talking about black grout or black mold on white grout? You can get it clean with a mixture of bleach and water in a spray bottle; let it sit a few minutes and rinse using a small brush. If you have colored grout, try using vinegar and water.

Thanks! Yes, black mold on white grout. I'll try that...

If you can find it, I like X-14 mildew and mold remover. They sell a soap scum remover too, so be sure to get the mold and mildew remover. Or use chlorine bleach like Clorox liquid bleach.

Bleach will eventually eat away at grout. A safer alternative is Oxi clean. I'd recommend you seal it after you get it as white as you want it.