I've tested on two different characters (Warrior Mage and Cleric) for when they can successfully forage for blue flower. Both were able to get them at 42 ranks during the day and 63 at night. This doesn't appear to be gradual but rather a binary function (repeated failures, the sun comes up and instant success).
I'm just not familiar with how to modify a wiki. I tried to update the table but could only put in one or the other value so I duplicated the table, one for day, one for night. If someone could fix it to one table I would appreciate it.