The sample in question (Trace B) tested negative for blood, as did every other sample taken from the blade.

IMHO irrelevant, Meridith's DNA is incriminating whether it's from blood or tissue, and whether any blood chemicals remained after cleaning in sufficient quantity to test positive.

Nonetheless, conditioned on Meredith's DNA being present, a negative blood test is surely significant evidence in favor of contamination over guilt, isn't it? If it was contamination, you would expect this with near certainty; whereas if the knife had been used to kill Meredith, what are the chances that any cleaning job was good enough to leave no trace of blood, yet bad enough to leave not only Meredith's DNA, but also granules of starch?

(Various lab procedural criticisms)

Do you want to formally introduce a hypothesis that, LCN aside, this test was sloppier than the average test at the average lab?

I'm not sure what you mean by "LCN aside"; LCN would seem integral to such a claim, since ordinary standards would already constitute sloppiness in the LCN context.

But anyway I'm not sure I need to introduce such a hypothesis, since my beliefs about the unreliability of the knife result do not particularly depend on any beliefs I have about reliability of results in the average lab. It suffices for my purposes that the facts mentioned by Conti and Vecchiotti are Bayesian evidence of contamination given general scientific "common sense". While I currently think it unlikely that Stefanoni's level of sloppiness is typical, if it turned out that it was, that would sooner undermine my confidence in forensic lab results in general than my beliefs about this case.

Formally, in other words, I view the base rate of contamination as being screened off by the detailed information about lab procedures, rather than an independent piece of evidence to be weighed.