Some more interesting info I found for knot bleaching. I am not sure if you have already read this but here is the link.

http://www.toplace.com/hairloss-info-tips.html#versus

Important info I found is that if the piece uses Indian Ramy hair then just bleach won't work. You have to use the fabric dye remover (apparently, this can be found at any grocery store) before you use bleach. Some more tricks also listed there. Let us know if it works for you!

Cheers!