Either the knife had bleach on it, or it had DNA on it, not both. A 2-3% dilution of bleach is recommended by Promega for cleaning pipets in PCR work. They also note that if one does not thoroughly rinse away the bleach, it will affect subsequent experiments. There is also a 1998 article in Biotechniques which shows that a 10% dilution of bleach damages DNA within one minute. If the prosection's claim is that the knife was bleached (and I am not certain that it is), then it is in conflict with their claim that the knife had Meredith's DNA when it was taken into custody.