I'm not a cop, but as a lawyer can answer your question.  You're correct.  Theoretically (because you never know once you get in front of a jury), the cop could be liable/guilty for civil/criminal negligence.  Civil negligence requires a) the defendant owe a duty to the injured party, b) a breach of that duty, c) causation (i.e. the cops failure to notify others was a cause of the woman's death), and d) injury.  A person in the cop's position owed a duty to Blake's wife to act as a reasonable person would under the circumstances.  A reasonable person likely would have reported Blake's soliciation to the authorities and he breached the duty by not doing so.  The causation issue is a closer one.  Since it is a likely that the wife's death could have been prevented by such a disclosure by the cop, causation may exist.

State's laws vary with regard to criminal negligence, so it depends on the statute in California.

...I for one hope it doesn't turn out to be another media circus like the OJ trial did!  Unfortunately, it's beginning to look as if there's a possibilty.  Curious thing caught my attention on the news, probably it means nothing & is just a trivial observation, but the lead investigator is named Ito.  Remember the dipstick judge that presided over OJ fiasco?

Or how about . . .

...that stroke of brilliance allowing a writer to mosey around an active crime scene, handling evidence?