Putting a blocking creature onto the battlefield doesn't trigger "When this creature blocks" abilities. It also won't check blocking restrictions, costs, or requirements.

The target creature can be blocked by other creatures.

Putting a blocking creature onto the battlefield will trigger "When this creature becomes blocked by a creature" abilities. It may trigger "When this creature becomes blocked" abilities, but only if the creature the Saproling is blocking had not yet been blocked that combat.

The Saproling token is blocking the attacking creature, even if the block couldn't legally be declared (for example, if the attacking creature has flying, or is Vindictive Mob).

Flash Foliage can be cast only during an opponent's combat phase, after attackers have been declared. If it's cast after combat damage is dealt, the Saproling will enter blocking the attacking creature, but will neither deal combat damage to it nor be dealt combat damage by it.

With the latest Oracle update, you can only cast this during combat, after blockers have been declared.

If a turn has multiple combat phases, this spell can be cast during any of them as long as it's after the beginning of that phase's Declare Blockers Step.