Whitening Dirty Washcloths?
Try All Fabric Bleach. A friend gave me some and it removed all the yucky baby barf stains. I was looking for an organic or biodegradable alternative to borax. Put about a 2 tsp in a bucket of hot water and let sit for a couple days. Wont bleach colors either...
Bleaching a Black Dress?
If you want to dye it because its faded then just redye with black RIT. If you want a white one take it to a seamstress and ask for a duplicate white dress. Try to bleach it and its toast.