Arsenic was always popular and unreliable to detect until a few years after your POD. There was a high profile serial killer named Sophie Ursinus who was caught poisoning several people with arsenic in Germany in 1803.
Poisonous mushrooms are a good staple as well. Charles VI was killed that way just a few decades earlier. The cause of death would quickly become obvious but since eating the wrong mushroom is a common occurrence it's something that can be blamed as an "accident". Even today lots of people die from poisonous mushrooms. Death's Cap is a good one to pick as it is frequently mistaken for a harmless mushroom.
Mercury poisoning would also work but it would take forever and more likely cause neurological problems and mental instability than death, although that might be what you were looking for. I'm not sure it would work as a purposeful poison though, since I'm unaware when mercury's unhealthy effects were discovered.
There's a very long list of poisons both current and historic that would be available to a member of a European court in 1790. If all else fails you could simply serve the king bad peaches since that seems to have killed more than one ruler.