There`s a thread here : http://forum.toplace.com/showthread.php?...+bleaching
Note ---- sometimes fabric dye is used to colour hair, and you can bleach the knots from now to kingdom come, with no effect other than heavy shedding, within a short time...... you may have to use a fabric dye-remover on the knots , if they won`t bleach for you --try a small inconspicuous area first [with bleach] and if they won`t lighten, you`re going to have to use fabric-dye remover on the knots before bleaching.....
Here`s another thread, with references to fabric-dye remover ..... http://forum.toplace.com/showthread.php?...ye+remover