As someone who doesn’t use Baneful Magick but is also absolutely fascinated by herbs, I’d say that an herb would only truly be “baneful” if it was being actively used in a hex- in other words, I don’t think there’s anything taboo about studying the properties of any and all herbs in the name of knowledge! 
On that note, have I got a book recommendation for you:
It’s called Wicked Plants: The Weed That Killed Lincoln’s Mother & Other Botanical Atrocities. I wrote a bit more about it in a book club review, but basically it’s loaded with all types of fascinating, deadly, and sometimes just plain naughty plants- it’s a very fun read! (A note that it is written from a botanical point of view, as opposed to a magickal one, so it may not be exactly what you’re looking for).
I dug around in the forum a bit and a few herbs with potentially baneful associations:
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Foxglove - a beautiful plant with mixed opinions. @Velle wrote about The Magical Properties of Foxglove and @Garnet shared Foxglove - Miracle or Posion?
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Lilac - some believe the lilac is associated with spirits (more in: Plants that attract shadows and spirits) and, when transplanted, will cause a death in the family
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Henbane - a dangerous herb associated with witches. @anon87969570 shared about it in Dangerous herb info
That’s all that comes to mind for the moment- I’ll be watching and learning along to see what others share too! 
Blessed be!