The origin of primum non nocere sheds some lights to its meaning.
Wikipedia gives a somewhat detailed story with reasonable-looking sources.
In short, the idea is ancient but the exact phrasing is quite new.
The Latin phrase primum non nocere seems to have been introduced in the 19th century.
The idea is that it is only the second law of medicine to do good, the first one is to not harm.
Therefore I would read the phrase more or less as praeceptum primum est non nocere, "the first rule is not to harm".
You could also read primum as "the first thing", but that has no real effect on the interpretation.
Either way, there seems to be an implicit est.
This reading is further supported by the fact that you can't really give any orders with non, so imperatives are out of the question — assuming the phrase was grammatical Latin to begin with.
A negative order should be given with ne or noli.
If you want to do something analogous about being blamed instead of doing harm, then you can simply replace "to do harm" (active) by "to be blamed" (passive).
They do indeed look like infinitives to me, not passive imperatives.
(Passive imperatives of non-deponent verbs are rare but do exist.)
Therefore my suggestion is primum non culpari.