I have searched and searched, but have been unable to find a description of the "poison" mechanic in Mojang Scrolls. I would appreciate an explanation.
Poison is used by a variety of scrolls in Decay and a few in Growth. All scrolls discussed are Decay unless otherwise stated.
Poison is an effect which can be applied to creatures (not structures). (An effect is a modifier attached to a unit which doesn't count as an enchantment.) It may be applied in several ways, but always has the same result. Quoting from the in-game description on a poisoned unit:
Units may become poisoned in several ways; directly by use of Ranger's Bane (Growth) or Brain Lice (note that Brain Lice applies poison and separately its own enchantment for the draw effect), or indirectly by the abilities of another unit (Vitriol Aura (Growth), Bitter Root, Blightbearer, Pest Dissimulator).
Poison damage is a type of non-combat damage. It is increased by Curse enchantments and ignores armor.
Blightbearer and Infectious Blight are scrolls which receive or deal poison damage but do not use the Poison effect itself; notably, Infectious Blight acts very much like poison but does not display the graphic for poisoned units.
Miasma Well is a structure which does damage to poisoned units only.
Poison is an ability that is currently only found in two growth enchantments: Vitriol Aura and Ranger's Bane, although as of writing the Decay deck is currently going through testing, and according to the Scrolls database will contain at least 8 other scrolls that utilise Poison.
The ability damages the target creature by 1 hit point at the start of each round for a specified number of rounds.
It's a bit hard to talk about scrolls that are currently in development, but as an aside I notice that Blightbearer will take "poison damage" before it attacks, which is essentially not how the "poison" keyword is usually used. I would hazard a guess that this is simply categorising the damage type, enabling scrolls such as Miasma Well to modify the effect.