It's mentioned in "Away With The Fairies" that cyanide has been used to tint photographs blue. In every episode of the series, the flashback scenes showing the murders are filmed with a blue filter, as in cyanide blue.
Fans of the show have complained of the decline in Jane's appearances over the series, eventually not showing in season three at all. It is mentioned in season two that she was attending school overseas. This is plausible because Phryne not only has the means to, but given her line of work as a lady detective (and the events ending season one wherein Jane was kidnapped by the same man who killed Phryne's sister) Jane was likely sent away for her own safety.
Aunt Prudence's fierce response to the fears of Mary, the former convent girl who was in labour at the time, that the nuns and God would take her baby away ("God will do nothing of the kind or he will be answerable to me!") was part Mama Bear instincts, but also maybe something more. Prudence dotes on her son Arthur, who's severely developmentally disabled, and won't hear of having him put in a sanatorium, despite her other son's irritable urging. It's entirely possible that Mary's predicament struck a nerve for her.
Hugh's assumption that Dottie will have to give up her job as Miss Fisher's companion if she is to marry him makes a lot more sense when you recall that Dottie's role as a companion requires living under the same roof as Miss Fisher. If Dottie tried to keep that position after marrying Hugh, Hugh would be required to live as more or less a permanent houseguest of Miss Fisher, which would doubtless be somewhat awkward for all concerned.
Hugh's quiet concern about being able to provide for Dot after marriage is much harsher knowing that their wedding is set just one month before the Great Crash of 1929.
It seems odd that Jane wasn't even mentioned in Modern Mysteries, and that Peregrine was revealed to be the sole inheritor of Phrynie's estate. However considering some 35 years have passed between the series, it's entirely possible that something happened to Jane. Either a falling out with Phrynie, or worse.