If you're looking into potions along the lines of things that would fall under the pop culture dark arts category, you can use any herb so long as you know how to use it properly. I would suggest doing research (if you do it online, cross reference the databases and make sure the information is consistent) to learn about the different uses of herbs. Specifically look into those that grow near you or are easily obtained.
i don't know anywhere where you can learn 'dark potions' but i think if you study potions and/or herbs you can make 'dark potions'. for example, some potions cannot be consumed because certain ingredients are poisonous, so instead they're to be worn, or the aroma inhaled, or poured somewhere [bath, lawn, running water] using the same logic you could make 'dark potions'. poisoning people by slipping it into their drinks i do not recommend, but you could make a potion with ingredients known to cause negative effects, and during the waning moon pour it onto their property to curse them. just an idea.
Hmmm, based on my perspective. If you would like to use herbs then basically if you are not going for that "way in too deep" thing then I would use a good "conduit" substance like sage or willow. For me since I am too lazy to actually make any malevolent effect THAT powerful, I would use a universal herb, like the quartz in gemstones. How I make these things is that I don't focus on the substance rather I focus on the charging. I focus more on the energies involved rather than the substance itself but correspondences are indeed important if you wanna get that extra "oomph" effect.
I would get some Willowroot/ Willow and make a ritual from scratch, I find the best rituals are the one's you make yourself but this proves to be universal to the eyes of those who practice magick, but I believe the BEST rituals are the ones you make at that moment when you are going to use it. Why? because in that moment or time span, the ritual you are going to be using is more in tuned to your energies at that similar moment you are going to cast it, rather than using a ritual made 5 years ago, You have the chance to renew and relate, isn't that the point of making rituals or spells? To relate yourself to it at that moment. At least that is how I perceive it.
I don't actually take a small time in making potions or casting in general. I basically take hours. As a High magician, how we first cast things is basically do a series of rituals that take excruciating long hours just to open a circle, like my personal favorite "The watchtower rituals" and adding some invocations to the proper deities to add some more punch and then channeling the energy through chanting. Then closing the circle, usually I would feel light headed so I tend not to do it too much and stick with moderation.
My tip is, if your starting out then you want to find a universal conduit for YOU, it may be a herb or an object, it can even be water since it's the universal solvent anyway. It is the energies you channel to it that actually produce the effect, the material things are just the Raw conductors, the better the conductor, the more effectively the electricity (energy) is being used. You can find those correspondences and "perfect substance" later when you are advanced, I would recommend Crowley's 777 for starters. It is straight to the point and gives you more than what you need.
A neat trick, if you want a portable potion?? or the sort, I recommend cursing a gemstone, whether it be quartz, obsidian,etc. It depends on you. I would curse this rock and put it in a bag that would "hold it" (Charge the bag so that the energy that is in it, stays in it!) then when I need the cursed rock to work, I would just drop it into the liquid that needed to be cursed and stir and say some words of transfer and VOILA! there you have it, a portable potion maker, it doesn't have to be a cursed gemstone, it can be a stone for healing as well, it depends on your will. Then drop it into a liquid and stir to transfer the energies. I find carrying a water jag full of cursed liquid to be quite annoying to bring especially if it spills.... YIKES!