Thanks for your helpful tips!A blue potion is 2 White Dragon Weeds, as long as you have the 'Dark Cauldron' ability you can make whatever you want even without the recipe book. The recipes you are talking about is on the counter of the clinic to the West, next to the scrolls, not Elise's room. That book contains Elise's secrets.
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However for some reason I still can't make the potions. Every time I put 2 White Dragon Weeds into the cauldron and Synthesize, I get an explosion.
I've tried this at least a dozen times already and the result is always the same.