The library is vague about these things, because faeries don't have super precise meanings behind things like flowers, or so it seems. Portland is a hotbed for fae flower imagery, of course, due to the Rose Queen's presence. Black roses, at least in human culture, usually represent death, mourning, or loss - which means that if the colours are intentional, it could be representing the death or loss of something. Of course, it could also be considered a serious threat, if you perceive this similar to the act of providing someone an engraved tombstone in advance of a fight with them.

All forms of the distilled rose petals will kill the mice, either quickly or slowly depending on how high the dosage is. The petals are almost certainly poisonous, which could conceivably provide more evidence to the notion that this is, in fact, a threat.