This is the forum for helping you out if you're stuck or if you want to discuss the nitty gritty details of the dungeons within Mount Grimrock. Warning: forum contains spoilers!
I'm down to level 9 now and I have never ever spot a single "Witch Flower". My guess is that it's simply the alternative name for one of the existing herbs. Probably Blooddrop Blossom, since that's used to make mana pots.
You're right! I just experimented: Mixed a grimcap, a bloodrop blossom and a cave nettle and get a Sulphurous Potion. That gives the status effect "Rage": Attack Power +10, Evasion -10.