There is nothing in the California Penal Code that illegalizes carrying any flashlight, either openly or concealed. The Penal Code that is most often used/misused when dealing with flashlights is PC 12020, specifically the portion dealing with "Billys". What bites a flashlight carrier is when they tell the cop that they are carrying the flashlight for self-defense. That right there indicates that the flashlight is being carried primarily as a weapon, and would then probably be identified as a billy club.
Never admit to any police officer that anything you have on your person that could be used to hit someone is being carried for any defensive purposes whatsoever. Exercise your 5th Amendment right to not self-incriminate.
My flashlight is carried in case I need to see something and it is dark, not for the purpose of being used as a Kubaton.
My cane is carried in case my knee flares up and I need extra support to walk, not for the purpose of being used as an Escrima stick, or for the purpose of being used to practice "Cane-Fu"!